well it seems that u f**ked up that windows. i dont think i will be able to repair anything as long u tried all possible. now u have to recover all data from the hard drive and make a clean install.
sp2 problems with bios??? never heard of this..... and u dont have to buy a new ram just ask a friend to give u one to try and then if that is the problem buy a new one.
also if u overclock and your drive is sata it might become unstable and geting errors. also scan your drive for errors from dos. or try with another cd/dvd-rom.