Could be anything from all the above reasons. If you want specific answers I suggest you describe what happened or you did before the problem occured(e.g. installed/uninstalled applications, visited websites, etc.,).

You could try to download and install HiJackThis:
http://www.spychecker.com/program/hijackthis.html
and post the result to try pinpoint the problem(baka makuha sa registry editing lang yan).

If you suspect a virus o spyware, try this:
http://www.trendmicro.com/download/dcs.asp
to fix eg. registry entries, services etc. made by known malwares.

O pede na rin upgrade mo OS mo na lang other than Window$ 98 (Microsoft has already stopped support for Win9X and WinME...)