12 year old pc is going to be slow (a cheap box from ebuyer..maybe £150..google: Zoostorm desktop pc... would be miles ahead of what you are using now), even without viruses and other crap clogging that's slowing it down.
But you could restore it to "as new" by just reinstalling your operating system, guessing it's XP?. That will also get rid of any crap viruses.
But that will erase ALL content, documents, videos, music and photos etc...so make sure you have them backed up to disc/flash drive/dropbox or wherever.
Another alternative is to run a completely free operating system like Ubuntu or Zorin...there are many other free linux out there. I myself use Zorin on a 10 year old pc and it flies!