The first possibility I can think of for the extra jumps would be that either your mouse or your keyboard might be tossing some extra bits due to a sticky switch or worn out cable or other connection.
If either of those is a "wireless" there may be a low battery somewhere. If they're on wire leads, the cable and/or connector(s) would be the logical suspects. A corrupted driver could also be involved, and the usual first action there is to go to Device Manager, delete the driver, reboot and let PnP reinstall the device.
A sticky key on your keyboard that tosses the Alt "bucky-bit" could cause you to jump back to the previous page when you hit either a backspace or left arrow key in IE. There likely are other "stuck bit" combinations. Your other browsers probably have the same or similar quick-keys as defaults. Cleaning (vacuum?) the keyboard might help, but trying a different keyboard, if you have one, is a slightly more reliable test.
Assuming that all of your "Antis" are thoroughly modern (current versions and signatures etc), common kinds of malware can be discounted; but there are forms (mostly root kit kinds) that aren't usually detected by any of those you're using. I don't have much on the few "special purpose" scanners that are sometimes suggested when there's good reason to suspect malware of this kind, although a couple of places specialize in the particular art form. Chasing this possibility can be really tedious, so it's common practice to eliminate all else first.
Erratic web behaviour can come from having you're clock set wrong. It needs to be within a reasonable agreement with the web clock that sites are using. With WinXP you should be able to double click the time at far right of the Start bar and find a "synchronize" button. It shouldn't make much difference which site you synch to. With recent futzes with daylight saving time this could be a common problem for a lot of people, but I don't think any of the deviant time-switches have hit yet.
You've probably already thought of all of the above; but I can't resist puzzling out loud.