I had the same issues you have when i was trying to get snow leopard on my AMD. i was about to give up. try this.
install retail from nawcom cd, once its installed (u said it boots right?) then go back into windows, and install the trial of macdrive. (i bought the program, its a GREAT program to have, it lets you access your mac disks from windows) reboot and voila your mac drive will show up in windows, allowing you to add or remove files. (including drivers(kexts))
find out what chipset you have (if its an AMD.. its either a Via, nForce, or AMD chipset).
go to
www.kexts.com and look in the downloads section at the chipsets. select the one for your particular chipset. keep in mind this is ONLY the IDE/SATA, northbridge and southbridge, or whatever, and NOT your USB or anything. drag the kext (make sure you open the .zip but NOT the .kext (as in windows u can open the .kext and it will look like text files). drag the .KEXT into the /Extra/Extensions folder.
Then, for your USB. i had good luck with VOODOOUSB. play with different ones. (also gotten from kexts.com. )
then go into the extra folder and open the com.apple.boot.plist file. if GraphicsEnabler is enabled (it will look like GraphicsEnabler then under it y.). either get rid of that line altogether (shouldnt say ANYTHING about graphics enabler) or put n instead of y. heres a SAMPLE one from my amd hack. i edited it for stuff that SHOULD get you booting and able to use mouse and keyboard.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Graphics Mode</key>
<string>1280x1024x32</string>
<key>GraphicsEnabler</key>
<string>n</string>
<key>Kernel</key>
<string>legacy_kernel</string>
<key>Kernel Flags</key>
<string>arch=i386 -v -force64 maxmem=2048</string>
<key>PciRoot</key>
<string>1</string>
<key>SMBIOSdefaults</key>
<string>Yes</string>
<key>Timeout</key>
<string>5</string>
</dict>
</plist>
when you save it, MAKE SURE YOU SAVE IT AS ALL FILES, NOT TEXT.
dont even worry about graphics or audio, we'll get it. i will help u as much as i can, since im off work for another week, maybe even 2.
when you install snow leopard, it SHOULD automatically find your chipset. (preselect.) if so, what is selected? uncheck everything else thats optional.... video, audio, wireless, (use the kext that works), rosetta, etc. if you can move the mouse and use the keyboard in the installer screen, then there is no reason you cant do it also within the operating system.
curious, do you know ANYTHING about your hardware? such as, what brand and model the motherboard is? basically at this point, all you need is the brand and model, ill google it for you and find the chipset type and ill find the kexts for you, if what i wrote above doesnt work. let me know.
edit: and if worse comes to worst, I turned a USB hard drive into a 10.6.3 universal boot disk. meaning i can plug it into ANY computer and it will boot into OSX. have tried it on intel AND amd. (AMD chipsets, VIA chipsets, nForce chipsets, etc). I will zip up the extensions folders (system/library/extensions/ and /extra/extensions/) and shoot them over to you via ftp or whatever. that will get your 10.6.3 install up and booting.