If you reboot, are you able to do ANYTHING before it starts, or does it do it immediately?
Major (pronounced mad-jewer™) things to try:
1) Does reboot help?
2) Does repair permissions from disk utility help?
3) Reboot from install DVD, repair the disk, does this help? Do you get beachballs when booted off the DVD?
4) If you create a new account, does that have this problem?
5) If you get this far on the list, and the answers are no, no, no, yes, then you probably have bad memory....
If you still have warranty, might be expedient to go complain to Apple.