I would tend to think that since you only have a 20GB HDD with three O/S's on it and all the other stuff you have installed and only have 128 of prob PC133 RAM and not a very quick prosessor that your computer is going to struggle and genraly be sluggish with every prosess it undertakes.

Maybe ditch an O/S, (a problem you might have is that say you have installed a driver under 98, when you boot up with XP the driver might not be compatable with it)

Or, upgrade you RAM, its fairly cheap at the monent.

Or, get a new CPU, you will prob need a new MO BO though, so this is proberbly the most expensive option.