Check your computer hard drive for bad-sectors/corruption.
Check your computer for virus/trojans.

If you find any such problems, format, replace hardware (if needed),
and reinstall the OS.

Default installations do not have such behaviors, so your system is
either corrupted, infected, or otherwise broken. Like an old car --
unless you know how to fix such cars, or the car is reliable, your
best bet is to swap out for a new car when you see signs of trouble.
Otherwise, you'd constantly be spending time/money fixing random
broken parts.

//David
http://w3-4u.blogspot.com
http://blogs.msdn.com/David.Wang
//