Nvidia geforce 8400gs won't let me open games?

ok, so my computer has kinda not allowed me to play games for a while now, but i finally figured out whats wrong, my graphic card. so after i discovered my problem i was relieved, relieved and then angry.... like WTF it's pretty much a new computer with a fairly new graphic card (i guess) and it can't play high def games.... kinda sucky.... anyways, i've been on just looking for answers to how i can chnge/fix my problem.... i haven't come across any, so can you guys try to help me come up with a solution? it's getting really annoying....... oh and have a happy Easter everyone!