>>4 indeed it does sound like an access rights problem, and changing that does requier some administrating knowlige.
but try the following:
reboot into safe mode, login as Administrator
go to your firefox folder (probably c:\program files\mozilla\firefox\)
now right click on firefox.exe and select properties.
There should be a tab namned 'Security' there, select it.
IF there isn't any tab named security, then press explorers menu item 'Tools' then select folder options, then click the tab View and deselect the item "Use simple filesharing (Recommended)". now click ok and then check if the secuity tab have appeard.
If it have not appeard, install winXP pro, or linux.
Now, check if your username appears in the list, if it does, click it and check if you have permissions to 'Read & Execute' the file, if this is in the 'Denied' list, try to change it to allowed.
If your name Dosn't appear, first check if ANY of the other names in the list have Read&execute denied, if so, change that and try to run firefox.
If it still doesn't work, go to the properties->security again and click the button 'Add'.
A new window appears where you can write in your username, do that and then click 'check names'.
If a new window poped up, you typed in the wrong username.
Now it nothing (except that xp changed your username a little) happend, click ok.
Now your username should appear on the list, select it and then select the option for allowing Full controll.
Click OK and then reboot your computer and login with your usual username.
and try to run firefox.
if it still doesn't work, format c: and reinstall xp or linux.