You cannot restore to factory settings without a Windows Installation disk unless there's a "recovery" partition on the computer. There are some tricks to speed up the computer though.
First and foremost would be disabling all the startup programs by going to Start > Run > type msconfig and press enter.
Go to the startup tab and press disable all.
Now re-enable things like the AntiVirus Software and critical apps.
If you don't know if it needs to run, just ask.
Next would be cleaning your registry. To do this, use CCleaner
. It is recommended that you do an inventory of all the installed applications and remove any that are no longer used or crapware (WebSearch Toolbars and the like).
When you run CCleaner, on the left hand menu choose registry and the click analyze.
Once complete, Fix the Selected Issues (All of them) and make a backup of the registry.
(When you click Fix Selected Issues it will ask if you wish to create a backup of the registry, click yes and save the backup)
Some additional tips for Windows XP
<General House Cleaning>