Dell Dimension E510 has Crashed!

my dell Dimension E510 has crashed, evrytime i start it up it asks to run in either safe mode, last know config, or normally. i chose either one and i get the unmountable boot error!!!! my computer has been running extremely slow b4 this happend, if i could get it back i would uninstall all the unnessceary programs like divx dreamweaver and fireworks, it has 3 different profiles on it.

 

the specs are as follows:

 

Pentium D 3.00ghz dual core

1gb of ram

basic GPU

windows XP SP2

 

i bought the computer from costco online about 1yr ago so it didnt come with the boot discs, how can i get the computer completly booted and running again???

 

ANY HELP APPRECIATED