While AGP write off or on really isn't a big deal, it could be indication of older/incompatible SiS GART drivers.
I'd go through the steps for a *clean* install, but ensuring to get the latest SiS GART driver (1.21 or 1.22 I believe) prior. I'd do the following:
1) Download, but don't install the full Cat 6.9 + CCC bundle and have it handy (desktop for example)
2) Download, install, but DONT RUN- Driver Cleaner Pro (
www.drivercleaner.net.
3) Download, but don't install SiS GART AGP driver from
http://www.sis.com/download/. Just click Windows XP, AGP (GART), then AGP (GART) driver. Download and leave handy.
Now, go into Control Panel, Add or Remove Programs and uninstall each of the following you may have, but don't reboot when prompted. Just keep on going until the last uninstall. Don't worry about any of these you may or may not have, just do in this order what you DO have:
Ray Adams ATI Tray Tools
GuidePlus+
ATI Remote Wonder
ATI Multimedia Center
ATI DVD Decoder
AVIVO Codecs
ATI Parental Controls and Encoder
ATI Control Panel
ATI Catalyst Control Center
ATI Display Drivers (click Select All, then ok.. then no to reboot)
ATI Software Uninstall Utility
After this, you may now restart the computer... BUT, restart in Safe Mode (press F8 after POST tests and select Safe Mode). Pick Administrator.
Now, navigate to C:, Program Files, Driver Cleaner Pro (or wherever you have installed this) and run it as administrator. Pick multiple cleaning filters and select ALL ATI entries. Then start the cleaner.
Once it completes, go ahead and reboot in Normal Mode as per usual.
Now-
1) Install the SiS AGP Gart drivers, reboot.
2) Install Cat V6.9 drivers plus CCC. Reboot.
3) Hit
www.microsoft.com/directx. Click the "Get latest version of DirectX" link and download the web installer. Let it run it's course.
4) Reboot once more, even if it does not ask you to.
5) On this reboot, launch the CCC and visit the SmartGART tab. See if you have AGP writes/reads enabled. Don't try to force anything- just note the settings.
It would also be useful to see if you're getting the 3d preview to work at this stage as well.
From here, we can help you install the multimedia stuff, tools/tweakers and the like.