puter no workie lol

well my computer was working fine till it started gettin errors with all of my web browsers then i got the blue screen of death "memory managment" even tho i have 1.5 gigs of ram. so i restarted it and then i get a screen at boot that say "no os system" im like wtf now , please help me geeks :cry

right now im using my pos etower 600is but ut works decent

my other machine that giving me the trouble is

emachine w2646
intel celeron 2.6 ghz ddr
1.5 gigs of ram

intel 3d extreme graphics agp
running xp home please help me

 

well i did it earler today and then i restarted a couple of times then it started up
then it was good then my browsers started screwing up so i restarted the computer then it now say no operating system