heres a wild guess.. dont hate me if im wrong
maybe ur accessibility messed up? if not try a new keyboard see if it is your keyboard. if not post back and ill try to help some more
sorry, my post wasn't the best answer but im a little clueless
edit:
ps
ur using 98 aren't u.. hmm ok idk if this is availible in 98 but here:
start>programs>accessories>accessibility>on screen keyboard
hit the tab button with the mouse and see if it does as it is supposed to, if not its prolly software if so then its ur keyboard