The problem is not likely from a virus. You said your hard drive isn't detected sometimes. The first thing you should do is get a new hard drive, and copy the old drive to the new one. The other thing to do is restore a good copy of your Windows registry. You can use a bootable cd such as ERD but if you have another working computer with a SATA port, it may be easier to work with if you connect your hard drive as a secondary drive. Change permissions to System Volume Information (add Everyone so you can access it). Pick a restore point from a couple days before it crashed. In your windows\system32\config make an "old" folder and move all the files in config into "old". Copy DEFAULT, SAM, SECURITY, SOFTWARE, and SYSTEM from the restore point to your windows\system32\config folder and rename them so they are correct. If it is connected as a secondary drive you can also run chkdsk and do a virus scan on it. When you're done put it back in the original computer and try booting again. If it happens again you can try an older restore point, or do a Windows Repair from your XP cd. Use a XP cd with the same service pack level if possible. You can use Nlite to update an older XP cd to a newer SP.
Issue resolved. Well, issue has been resolved and somehow, I managed to bring the computer back on track. What I did is somehow managed to get my WDC HDD detected and booted my PC with Vista DVD. Than, I clicked on Repair my computer at the second screen - clicked on Load Drivers and than I got complete access to all the partitions on my HDD. Therefore, I immediately copied (overwrite) following files from C:\WINDOWS\repair to C:\WINDOWS\system32\config and vola! My PC was back on track. System, Sam, Security, default, Software, SysEvent.Evt, System.sav Now, I am getting all the softwares back in place, the configuration for which no longer exists in the registry entry b'coz of all the above files getting overwritten. System is working smoothly. Once everything is in place, I'll copy all the files from C:\WINDOWS\system32\config to a backup folder on C drive to make sure that the next time, if I face any kind of similar issue, than I already have proper system files and settings in place which can be used to overcome the issue. Regards, Apoorva.