System: ECS RS482-M motherboard with latest BIOS, 1.5 GB RAM, Athlon 3700+. The motherboard has low-level built-in ATi graphics, but I was running an nVidia 8400 GS graphics card. The machine worked very well under both the Windows 7 beta and then the release candidate.
I wanted to upgrade the graphics a little, so I bought a cheap Radeon 4650 card from Newegg.com. The brand is HIS.
Unfortunately, with the new card installed, the machine runs painfully slowly. (I.e., boot-up takes ~ 5 minutes; it takes forever just to do something simple like open the Device Manager, etc.) The Task Manager indicates that the CPU is pegged at 100%, but none of the running processes account for this. The slowness seems to occur from the instant the Windows 7 start-up process begins. The driver for the new card has been installed, and is supposedly "running normally," but it doesn't make any difference in how the machine is running. I've booted in Safe Mode, with almost the same result (CPU running in the 90+% range continuously for no apparent reason, machine running bog-slow), and I've tried some of the various other options you can get by hitting F8 at startup; nothing helps.
If I remove the new card and put the old nVidia one back in, it reinstalls itself and everything is fine. If I start up with nothing in the PCI-E slot, and the monitor hooked up to the built-in graphics connector, the built-in graphics installs itself and everything is fine. Either way, if I then re-install the Radeon card, the problem returns.
If I remove the hard drive I have Windows 7 installed on, and replace it with my XP64 drive, the 4650 card works fine. (When I first installed it, the 100% CPU thing was happening, but after the driver for the card was installed and the machine rebooted, it was fine.)
I'm stumped. Is there anything else to try, short of re-installing Windows 7?