Yes you can format and install XP from the original CD you will then need to download and install all the updates.
Also you will need to reinstall all drivers and programs.
This is why format and start again is last option when trying to overcome problems.
1. uninstall all no longer used programs
2. run antispyware and antivirus progs to get rid of any nasties.
3. run something like Crap Cleaner click here to clear out all temporay files and fix issues in the registry.
4. defrag and scan disk the HDDs
and you will be suprised at how much faster evey thing ten runs.