I recently set up a Windows Vista HTPC and only had 512 MB memory and a Sempron 3200 lying around. Until I could get to the store and pick up another stick of memory, I thought I’d try it with 512 MB RAM. Whoa! Vista was SLOW. I spent a few hours tweaking to get it to work faster, and to my surprise it worked.
I’d have to say the #1 tip for speeding up Vista when you have 1 GB of RAM or less, is to disable Superfetch. Superfetch memorizes what programs you use and when, and loads them into memory during start-up. This is a fantastic idea iff (if and only if) you have large amounts of memory. If you have 512MB or even 1GB in some cases, you may be better off disabling Superfetch. This will free up system memory and allow it to be used exclusively by the program currently running (good for games too).
To disable Superfetch go to Start -> type services.msc in the run/search box -> Double-click Superfetch -> Set Start-up type to Disable and press Stop. Then restart your computer.