Memory issues for Vista

So, I'm not sure what the heck happened, but Vista recently has had issues with a lack of Memory, but the same amount has been used since the get-go. It averages around 75% and 80%, but lately it complains that there isn't enough Memory and wants me to close certain programs, the ones I am mainly using and it's getting annoying. I've run Spybot and then downloaded Kaspersky and installed it, which got rid of Spybot btw, but it found trojans that avast! didn't find. It got rid of them, but they may have been around before it started happening. I thought it was because I installed SP1, but got rid of it and it started working slightly better. I also thought it might've been contributed to the newest version of iTunes, which I've been told has been known to ruin computers? I don't know, but that's what I've been told. I have no external currently to back-up all my stuff and just get rid of Vista and go back to XP Pro, but I wondered if there was anything I could do to get rid of this problem until I can actually afford my external and not have to deal with it constantly crashing?

how much ram do you have installed?

buy more ram an install it, it will fix the lack of memory problems.