I have just responded from another thread, Nathan pretty much had stated what need to be done.....Vista does not run too good with only 512 MB of ram.....
By the way, I still have a Acer Aspire just like yours (my kids use it) and yes, it was a bit slow with Vista, so I added another 1 GB stick of RAM and sped it up a bit. I was able to try out the beta version of Windows 7 and it seems to be speedier than Vista. I think you also need to turn off the sidebar if you plan to keep only 512 of memory.
You might want to try this all in one solution...
Tweak and Optimize your Windows Vista - Vista Manager
I have tried it before and it seems to work sufficiently......it has a 15 day trial to get what you need done....
