Internet is causing my iMac G5 to freeze

Wednesday morning I turned on my computer as usual, logged in, and it began to load the 3 programs it always loads automatically: Excel, Firefox, and Entourage. Before it was completely loaded, the whole comptuer froze and I couldn't do anything - even Force Quit (I thought the problem was with Entourage at the time). I had to hold & press the start button to restart, but then it wouldn't even get to the point where I could even log in. Since then I've tried logging on to the two different admin accounts on my computer, but sooner or later the computer just crashes. I've tried using Safari instead of Firefox, but it still freezes. It's okay if I only use Excel or something that doesn't need the internet, but there's definitely something major wrong. Is there a virus out there that is coming from Firefox (or even Facebook, since opening FB seems to make it freeze all the time now)? Have there been secret upgrades to something that could be affecting this? A couple months ago a Quicktime update messed my computer up, but I don't remember telling to do any updates. THANK YOU for any help. Robyn

There is no virus Robyn. If you can get the machine running, uninstall the programs that are booting on startup by going to System Preferences>System>Accounts>Login Items and deselecting.

If all goes well boot from your install DVD by holding down 'C' immediately after the chimes, run Repair Disk from Utilities>Disk Utility, reboot and run Repair Permissions and then update with the OS X.5.6 combo updater if not already done.

http://support.apple.com/downloads/Mac_OS_X_10__5___6_Combo_Update