I assume you have tried the usual virus update and scan, etc?
Otherwise, take a look in the system log. On XP, go to start, settings control panel, administrative tools, event viewer
Then look in Application or System, see if there are any relevant Errors (not all errors are actually anything to worry about). Double click on them and read the description, see if that helps.
Feel free to PM me what it says when you click 'view technical information' on your second image.
Are you using SP2? (...I'm assuming XP here...)
...and try Firefox again. Use it for a week (have IE there to fall back when sites dont work, there are really not many) and I guarantee you'll not go back