I've found that Desktop searches, unupdated graphics card drivers and anti-virus or anti-spyware programs can all be problems. Also, close unneeded processes before starting, defragment your hard drive and check that there is a few GB of free space on it.
My computer has only 256MB RAM and a single 2.8ghz processor, running Windows XP professional and the Sims 2 runs. Slowly, but it runs.