Windows Xp

I have a compaq Presario that came with a lot of useless programming I may have inadevertently deleted a needed program what can I do to correct this?
Every time I start my computer it says it has an NDA Error which reads
core.hp.main/application.xml I signed up for Road Runner High Speed Internet
but for some reason cannot access it from my computer.I am haveing to rely on my ex wifes computer to access via road runner with in the same room hers seems to work fine.Could the before mentioned NDA Error be the prob. Help do not know alot about computers but I am tryinrg to learn.Thanks

 

You could restore the OS with the recovery CD that came with the computer, if you were lucky enough to get one.

 

Did not recieve a recovery disc with my system.My ex has windows XP and a compaq presario also it does have a recovery cd burn option would it work if I burned a cd off her PC and used it on mine? If so is there anything special I need to do as not to loose other files already on my PC? Thanks