Unable to install Win 7 - clean install

I'm an IT student who got a free Win 7 x64 disk from my school, and I am unable to do a clean install. My system specs are as follows:

Intel Core 2 Duo 3.12 Ghz
nVidia GeForce 9800 GTX 512 MB PCI-e
320 GB Western Digital HD SATA
1 Gb DDR2 1066 MHz and 2x2Gb DDR2 1066 MHz SDRAM

I set my boot sequence to boot from DVD-ROM drive first, and HD second. It goes to the "Windows is loading files" screen and gets hung up. I started the process and went to watch TV for an hour, and it is still on the "Windows is loading files" screen. I haven't gotten any error messages. I've unplugged all USB devices from my computer as I have seen on this forum that it has worked for some people.

Then, I got frustrated with the process, so I tried to go back to Win XP and it said my bootMGR is corrupt. So, now, I am unable to install Win XP and I can't find any answers as to how to fix my problem on the internet.

Please help!

edited to add: I originally had only 2 Gb of RAM, and it got hung up at the "Windows is loading files" screen. I thought it was getting hung up because I didn't have enough RAM, so I bought brand new 2 sticks of 2 Gb RAM. I took out one of the 1 Gb sticks and placed both of the 2 Gb of RAM...for a total of 5 Gigs. And, it is still doing the same thing.

I removed all but 1 x 1Gb RAM, and it installed with no problems. Now, I would like to know why removing extra memory will make Win 7 install properly? Will I be able to install the other RAM now that I have Win 7 installed and working without anymore problems? Or, will I need to upgrade to the x64 version?

Btw, thanks for helping me get this problem straightened out. I very much appreciate your help.

Switch of, and install the memory a stick at a time, or put it all in and remove one at a time until it works. I think you just need one in to get it installed, now that you have, it's time to reinstall more memory.