Computer dramatically slow with Vista

Hi,
I installed Vista on my PC (ACER Aspire T120E-S965 with AMD Sempron(tm)
3000+ 2.00 Ghz and 1.00 Go RAM).
After trying more than 10 times, it seems to be finally stable but it is
DRAMATICALLY SLOW!!!
I'm glad I chose to install it on a second partition, using dual-boot
because I couldn't use it for my general work in such conditions.
With XP, it works just fine (so I don't believe my computer is responsible).
I also installed Vista on my notebook which doesn't seem to suffer the same
problem.
The major difference between these two installations is that I activated
Windows Vista on the slow machine and not yet on the notebook.
Any explanation?
Thank you