Tell us a bit about the problem... is it hardware or software based? If hardware, I'd recommend a local, privately owned business - you'll get better service with some personal attention and who knows what perks. Could also work for software problems, but Windows has great "self healing" utilities.
If you want an online program... http://www.pc-doctor.com/
or http://pc-system-utilities-software-review.toptenreviews.com/advanced-system-optimizer-review.html
Look pretty good... Haven't used either myself, though. You'd be wise to research it more... iolo's System Mechanic Pro also looks good. I use regular System Mechanic. There's a deal out on that, so google it.