A new user account does nothing to help. It is still super slow
Gotcha.
So you have a systemwide issue then (not just a single account).
Did this start happening after you installed some new software or maybe an Apple Software update?
Did you add any new hardware to the computer like RAM or an hard drive?
Are you running any Anti-Virus or Firewall software on the computer (doesn't come with the Mac so it would have had to been installed by you after the fact)?
_Turn off your Airport by going to the airport menu and choosing Turn Airport off.
_Unplug all external devices besides the power cable.
_Restart the computer. -- Any change?
If not:
_SafeBoot the system and while in SafeBoot remove the Macintosh HD > Library > Caches folder to the trash.
_Remove any items in Macintosh HD > Library > StartUp Items
_Reset the PRAM:
http://support.apple.com/kb/HT1379
_Any change?
If the issue persists at this point I would backup your data and reinstall the Mac OS (with 10.6 this does not erase the computer, it just puts the OS back on the computer):
_Insert your Mac OS X 10.6 installation disc
_Shut down your Mac.
_Start up your Mac while holding the C key to start from the disc and follow the onscreen prompts to reinstall.