I have installed Windows 7 on an old Athlon XP machine with the following specs:
CPU: Athlon XP 3200
M\B: Abit AN7 nforce2 ultra
RAM: 2*1gb Corsair
GPU: Leadtek geforce 7800gs.
HDD: Maxtor 80gb SATA
DVDRW: some IDE model.
The system is running just fine with all the drivers installed and I can browse the web, play videos and mp3's with no problem. The only major hurdle is the lack of SSE2 instruction set that prevents many software form installing. I must say that the same system with Windows XP SP3 POS updates installed is running much, much better: all the benchmarks are scoring at least 25% higher and the system feels more responsive. I will post screenshots when I get home!