My mother-in-law's Windows Vista desktop ([Dell Inspiron 531](http://reviews.cnet.com/Dell_Inspiron_531/4505-3118_7-32486097.html)) is running slowly, and I think it is because it is running low on memory. When the computer has been sitting idle for a while the physical memory usage will get up around 90%. The UI is laggy, usually it takes about 10 seconds to see any effects of a click, and opening a Firefox window will take a full minute or so. After using the computer for a while it will gradually go down and stay around 60%, and at that point the computer becomes responsive.

As far as I can tell there is no single process hogging more memory than expected. Obviously the browser is using a lot (~100 - 200 MB), and the next in the list is the `svchost.exe` instance that's part of the Services process including stuff like SysMain, Netman, hidserv, wudfsvc, UxSms, etc. The computer is up to date, and she's doing regular virus scans with Microsoft Security Essentials. She doesn't have a ton of extra stuff running, but there are a handful:

* WeatherBug
* IncrediMail (I can't convince her to just go to GMail....)
* Lexmark Imaging Studio
* LogMeIn (so I can do remote maintenance)

She has 1 GB on a Vista desktop, which seems reasonable, and it was fine last Christmas. I'm more experienced w/ UNIX, and while that is a bit low, 1 GB of memory wouldn't be prohibitively slow like it is on this machine. The "Performance Information and Tools" section of the control panel rates her processor at 3.4 and her memory at 4.5 (out of 6?).

So what can I do to make the machine run faster? Is it possible I am missing something that's hogging memory and causing it to run slowly? The only steps I've taken so far is to disable unused startup items, disabled all visual effects (really, this computer looks like 1998...), and run disk cleanup (she's only using a third of her 150 GB). I'm about to break down and purchase more RAM, but I wanted to check just in case.