|My personal views:|
As riider point out; this is a very old PC with a capacity for only 4GB RAM - max.
No reason why you shouldn't eventually be able to get it running reasonably sweetly, but given its age and outdated architecture, it's never going to be fast, nor is it worth updating to a later version of Windows.
If it's a hobby project, then fair enough, but if it's for everyday use on the internet, don't spend too much money on it, given that XP will become unsafe to use online after April 2014 when Microsoft end support.
Re System Mechanic: I see the latest, 'Pro' version includes an antivirus/antimalware function. Personally, I doubt its efficacy in this department. There are better/safer, AV and other 'clean up' programmes available.
As for the ability to fix/speed up any PC:
As has already been mentioned by other contributors, such tools cause more trouble than they're worth. Indiscriminate running of such 'tune up' programmes inevitably causes major problems down the line.
If this is a trial, then uninstall it. if you bought it, then be careful how you use it.