Hi and welcome to the forum
Only recently computer producers stopped to include a recovery disc in their products, but in 2005 for what I know a recover CD was included.
Do you have these discs?
Bear in mind that using restore CDs will remove everything you changed in your computer, so a backup of all important data (like e-mail accounts, saved pictures, etc) is mandatory.
However, an OEM operative system (be careful to purchase from ebay: choose only reputable vendors) can be installed in every computer, unless is an highly customized version purposely made for a specific machine (like restore CD sold with a specific computer model).