Try to make sure they dont just wanna sell you a new hard disk, also do you have a win xp disc or recovery disc ( a lot of the big companies have a win xp disc made specially just for that pc) If they give you a quote let people on here check it out for you.
Many pc repair places i get told suggest the disk is corrupted (ie no good) and can't be used again ( occasionally happens but not as often as people suggest trust me)
If i was there i would first try to boot in safe mode (f8 on most laptops or pcs and gives a menu of options) if no good would try booting with a special disk (booting starting the pc from a disk) with antivirus and other software.
I would hope to recover the special files, depending on the virus this may or may not be possible.
The other possibilty is if you have a bsod then some hardware adding is causing the problem then disconnecting everything but the montior, keyboard and mouse may help if you try to load in safe mode.
If you can boot up and get to a windows screen
press ctrl+shift+esc this will display the task manager
this can show you various bits of
info but will show what programs are running in many cases the easiest way to find a "bad program" is to see it running. Then just search for it in file explorer and delete.
A friend had a program eating up his hard drive space he let me on remote check his task manager we found a strange program we deleted it using hijack this and it was still there so belive it or not we did a search for it and found this huuugggee file which we simply deleted it. It never came back. He ran alsorts of programs in the end we did it the simple way.