Why has my iMac gotten slower for the past few months?
Here's a few things I would try:


Do basic system maintenance (repair permissions, clean caches, run daily-weekly-monthly maintenance scripts). I like ONYX (http://bit.ly/eCzYfj) for doing this, but there are plenty of other options that do the same thing.
Make sure you have at least 20% free disk space. If not, try clean up and free some space.
If things are still slow, try creating a new user account and logging in with that. If that account runs much better you still have some cleanup to do.
Check your login items (in system preferences, User &amp; Groups). If there are login items you don't recognize/need try disabling them.


I would also look into investing in at least a 2MB memory upgrade. Not too expensive and it looks like you could use it.

Good luck...