Did you install drivers when you changed your wireless network ? undo what ever you did. Try removing the wireless card and if that lets you get into windows then use system restore or remove drivers and try wireless again.

Go get Memtest 86 and read the directions, it's easy to use. I like the ISO file burn it to CD and in your bios set first boot to CD drive, the floppy file is easy also. Test your memory.

Are you over clocking your system ? back off the OC if you are.

Usually the problem you having is caused by a bad OC, bad driver or bad memory.