Serious crashing problem

My PC used to work fine, but last year, it started to become a real pain in the ass. The problems started when I tried to boot it up one day, and it came to a blue screen error that said at the top "UNMOUNTABLE_BOOT_VOLUME", and it refused to boot to Windows. I had to reformat it, and I would try to reboot again. My PC would just repeat this after I had left it off for long enough, or sometimes when I would only restart it. So, I have ended up formatting this pile of junk 16 times in under a year just to keep it accessible. I've tried everything - Virus scanners, trojan scanners.. Nothing fixes this issue. I've tried Avast!, AVG, Spybot, The Cleaner, Norton, AdAware, Registry Mechanic, and many more "repair" programs that never did a thing to help, including Hijack this.
Sometimes the PC would lag really badly whilst booting up. The problem occured again tonight when I tried transporting my PC an hour from one house to another in a cold car trunk, but when I took the PC to the house days prior in the warm interior of the car's back seat, it booted up properly. Could cold weather have a negative affect on it? Could I have a boot sector infection? I don't have any hardware failures that I know of. My computer is a Dell Dimension 2400 desktop with 700 (something) ram, an intel extreme graphics card, 36 gig harddrive, Windows XP home edition and a CD-RW drive.

 

Computers love cold weather. Are you saying that when the computer is cold it doesn't run properly?

 

If it's giving you an Unmountable Boot Volume error, see if THIS helps.