I have 512mb of RAM on my desktop. I upgraded it to 1GB. It worked fine for a day then it crashed. When i restarted it, when the "windows is starting up screen" appeared, it restarts. It also des the same thing when i put the 512mb stick back in Can someone help me fix this problem

have you tried booting into safemode?
If it does the same thing then you will need to boot to your instal CD in order to access recovery console to run chkdisk or maybe SFC.

If you can boot into safemode, schedule chkdisk /r which should run at next restart, and reboot.

Do all this with the original ram only.

After you get your system booting normally, disconnect the harddrive, insert the new RAM and run Memtest86 from a bootable floppy or CD to verify the ram you bought isn't faulty.

Goin' Fishin' (Some day)

If that fails, try one of those live CDs. Mostly they are Linux but you can get Windows ones too. [don't understand Linux: doesn't matter. We are only trying to see if it works).

If that works, then the motherboard's OK. Sounds like a hard drive fault.

I would recommend a recovery with Spinrite from www.grc.com.

Terry@nz