defragging is only needed if the game swap shitloads otherwise its useless to defrag.
and with 512mb ram as he has it wont be happening.
i rarely defrag and my games runs smooth as milk, memory is the key word here
you shuld get better performance in your games, the kt400 chipset is not a monster but it is for sure enough for games (i have 1 my self).
try different drivers for your card.
try other drivers for your motherboard, there have been quite a few hyperion drivers for the via chipset.
check sound drivers.
remember though, mohaa really runs like crap on lots of systems..poorly optimized.
i have heard the same shit about raven shield too.
try some other games and you will know for sure if your system is major f**ked up or if its just those 2 games

try the call of duty demo for example, same engine as mohaa and the expansions but runs miles better.