If you do not have a recovery disk ..the fastest way and safer and much reliable is to reinstall it clean from a Boot-able disk with the operation system ...
After installing the clean operation system ..all you need is to get the drivers for the devices..(sound; video ;Lan card;//etc) from the HP support site ..and reinstall them for to be working proper..
During the installation you will have to format the Cpartition in order to get rid of all the viruses for sure ..
Here you will find an image file with a boot-able windows XP..that you will have to download using Utorrent or any torrent client software ..and after that burn it on a blank CD using NERO burner software ..
The actual burning is made just by double clicking the file with a blank cd inside..and the Nero program being installed on the computer ..
Here are the links in order for ..windows...utorrent ..and Nero ..http://thepiratebay.org/torrent/5388204/Windows_Xp_Genuinehttp://www.utorrent.com/http://www.mediafire.com/file/yznzmdonjgm/Nero%207.rar
just install utorrent first ..than download Nero and install it ..(be sure you insert the product key in the folder) ..and than download the windows xp image file from that link ..
when all that are done and nero installed..insert a blank disk ..and double click on the image file ..it will bur a boot-able windows xp ..
with this cd you will be able to boot from cd and reinstall the operation system ..
after that here you will find the drivers you need for your computer ..http://h20000.www2.hp.com/bizsupport/TechSupport/SoftwareIndex.jsp?lang=en&cc=us&prodNameId=316715&prodTypeId=12454&prodSeriesId=316713&swLang=13&taskId=135&swEnvOID=1093