Without CD's, if the Original Windows XP that came with the computer is the one installed, you can completely format and reinstall the Operating System with all the default software that came with the computer suing the System Recovery Option built in to the hard drive on the recovery partition.
Right after you power your computer on, start tapping the F11 button. After the Compaq splash screen disappears, the computer should take you to the System Recovery screen where you can choose to completely Recover your computer back to system defaults. I tried to condense the steps listed on HP's website so they wouldn't be asdf confusing. Here's the link for all the recovery options: http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=115&prodSeriesId=1847962&prodTypeId=321957&objectID=c00608578
Go to the section named: Reinstall the operating system using the advanced recovery options
Here's the basic Recovery Steps without disks:
Plug in AC Adapter
Power on your computer
Right after you turn the computer on, repeatedly tap the F11 button
After the Compaq screen disappears, the System Recovery Screen should appear shortly after. Stop tapping F11 when you see this screen
Choose to do a System Recover and click next
Check if next screen has "Advanced Options" to choose from near the top.
If it doesn't, click next, and hopefully the next screen will have "Advanced Options"
Click Advanced Options then choose Destructive Recovery
This will do a FULL FORMAT of the computer and reinstall the Operating System and default software that came with the computer. (Note: this will wipe out all data you have saved on your computer)
Click Next with Destructive Recovery selected.
Reboot the computer when the computer prompts you to reboot
Process may take approx. an hour I believe. DO NOT shut off the computer while the process is running.
Hope this helped, read up about it on HP's website from the link I provided and post back and let us know if this worked for you.