Won't startup

Won't load Desktop
Won't load password window
Tried warming in sun prior to booting. Booted and ran fine for 2 weeks using sleep mode at night.
Ran fix permissions. Many fixes.
Ran disk repair. No problems found.
Turned off. Back to won't load password window.
Booted from install disk.
Tried to run repair permissions. Immediately quit repair program.
Ran disk repair. No problems.
Rebooted using cmd+s. Ran fsck -f
Error message: File system was modified.
Ran the rest of the disk checks. Volume OK.

The computer running fine after warming sounds like a hardware problem. Inability to repair disk permissions and "File system modified" sounds like a virus.

Any suggestions.

It's nice that you reported all you have done.... too bad you didn't tell us which year model Mac you have and what version of OS X it's running.

Try this: Remove the hard drive (assuming it's a MacBook or MBP). Try booting with your install DVD after the hard drive is removed. Post back results and specs for your Mac.