Back in trouble

Hello again,here I am once again bending your ears,YES you've guessed it,I have a problem.
Whilst on my PC yesterday the Screen turned BLUE with the the Windows (Microsoft) logo on and started of its own violation to do some kind of a disc check,and removed a lot of "Corrupted Files".Now I've discovered that I cannot load ANY of my Store Bought Sims including the ones from UKTS,though I can install ones copied from there (UKTS),having no trouble at all in installing the Woodhead and CVR.Any ideas would be gratefully received.Its very probably something to do with something I have tried to install,as once again I have failed miserably trying to download Stock items.
Obviously this is a pain,but not the end of the World as I've been promised (sort of ) a new PC from Santa. Kindest Regards

Hi Ged Hello Mike,I am one of the few still using XP,my MSTS is on my J Drive.As for what I was doing,well I was trying to turn off a game  that one of my grandchildren had been playing,it had frozen and the curser had disappeared so I tried Ctrl/Alt & Delete which usually allows you to stop whatever is running.But in this instance that did not happen,but a complete row of green boxes appeared across the bottom of the screen,which then turned the whole screen Blue.My PC is at least 10 years old,so I suspect it will be a hard drive fault,I have been putting off getting a new PC for quite a while after borrowing my sons (Windows 7) for a week and finding it not really to my liking.Still needs must that I get a new one but have not heard anything good about Win 8,so it will have to be a Win 7 which are luckily (I suppose) still available.
Thanks to you both for your speedy replies,which have confirmed what I expected.

Hello everybody ! as far as I can ascertain I am running Windows XP Home Edition (5.1 build 2600) service pack 3. My Processor is an Intel  Celeron  D....CPU 3.06 GHz with an AMD RADEON HD 6450 Graphics Card !
I'm sorry Ged but I haven't got a clue what a DOS Box is,but I assume that its what I use to get CHDSK,but I do that by Start-My Computer-(right click on ) C drive- Properties- tick the 2 boxes(Scan and attempt recovery of bad sectors & auto fix)-Restart PC and it does the test.    TWICE after I have done this  my PC has failed to proceed past the Welcome Screen forcing me to Manually Stop the PC,restart while pressing F8 and select "Restart the Computer as at the last time it worked correctly" (not sure if that's the exact wording).
35005CP Andy I am using "Avast" anti virus and "Advance System Care Ultimate" for Malware etc,I have always used C Cleaner.As for the Operating System I did not realise it had to be reinstalled,and wouldn't know how to do so.When the fault ? occured I was not downloading anything my Grandson had been playing some online game and the PC completely Froze Out.
As a little add on I said earlier that I cannot download (install) from any of my discs,that's not quite true as I've completely uninstalled MSTS,checked in the files that it was gone and it has reinstalled perfectly,Yet my Cambrian Route bought on CD from 3DTS (quite a while ago) nor my UKTS Thames/Mersey (Cd) make any attempt at loading.
Many Thanks to you all,Kindest Regards.

 Firstly let me thank you all very much for your help,but I get the feeling I am digging an even bigger hole.
Ged in answer to you "No it does not run in a Black box"the whole screen is blue with white writing,But if I follow the instructions from Andy it does.,chkdsk then runs as you said it will and says it is -"deleting index entry320070100000009.bin in index $ 130 of file18905". If I then ask it to do a repair the DOS box appears for a split second only,No message appears asking me to do a restart.
Alec your idea appealed to me so after doing the above I decided "what the Hell"and gave it a go,Sadly when it had restored there was no MSTS or Train Store present though it was definitely there  (or should have been) at the time to which it was restored.Looking now where MSTS should have been there is only one file (which is empty) named "Program Files (2) which I'm a bit worried about deleting as the name is similar to where all my Programs are filed on my C drive.
Finally I do have a Dell Re installation Disc for XP SP2,but when I put it into my loader the message came up that "this is an older version than the one in use,and cannot be installed without first rebooting and restarting from this disc.Note that most files will be lost should you proceed."
PS I tried to put an image on but without an IDIOTS guide on how to do it,I've failed.

  Firstly I feel I must thank you all for your very generous assistance,especially being as I really have no way in which to repay it being completely PC illiterate.
I tried using the Dell Reinstall  Disc,but apart from getting the info screen it refuses to fire up,just like all of my TS add on's,except for the MSTS disc itself which loads normally.
I did get a screen shot of the DOS but cannot seem to get it onto here (TSSH)
Anyway I digress,after doing Chkdsk/Repair again (3 times now) my PC is starting up as it should and I'm wondering as to whether or not its some sort of firewall thing or even anything to do with ASC Ultimate (maybe its to ultimate)so I have turned down the settings especially the ones that "run in the background" and some of the things it did when you first turned on the PC which meant turning on and being able to use was running at 6/7 mins:
Having reinstalled MSTS again this morning I have put LSE back on,and it has gone on and is working even though when I put it into the drive the screen came on for me to enter the code when I asked it to load it just ran and ran,then came up with "NOT Responding"so I opened the drive-got the warning Beep Beep - re closed the drive,clicked on continue at which point another box appeared in which I again put the code,clicked to download and the box just disappeared to my screensaver.Assuming that was it I went and had a cuppa and upon my return some 10 mins later there was the loader screen waiting for me,clicked on it and away it went and loaded.
Of course I do not for a minute think its cured,but as My Missus is threatening to get me a new PC for Xmas I,ll be happy just for it to run until then.
So once again a BIG thank you to all,I'm off for a quick trip to Sevenoaks ! I don't expect to  arrive on time.