Did you update you machine to Vista or you bought a new machine with Vista?
I asked because Vista is the biggest "memory hog" of them all - requires 512 ram for OS alone. If you upgraded OS, you might need to go out an buy more ram.
Also, the memory problem is just the tip of the iceburg, when it comes to Vista ( read
http://www.slashdot.org) ... you probably are better off waiting for "Vista Sp2" or what ever they will call it.