I would suggest that you manually do your own basic tune ups with software that is provided by microsoft. There's very little chance of stuff going wrong if you just do the basic stuff.
Here's what I would do:
1) Create a system restore point (if you don't know how, just google it, its very simple)
Start>All Programs>Accessories>System Tools>System Restore
2) Go through your list of install programs and remove anything you don't use/dont recognize.
Control Panel>Add/Remove Programs
3) Clean up your computer start up routine
Start>Run> msconfig
When the program opens (System Configuration Utility) click on the start up tab. Go through this list and uncheck any program that you do not require running all the time. The only thing that I normally leave on here is things like my antivirus. Everything else you can launch when you need it.
When you're done going through the list and unchecking things you don't need, then click okay and restart your computer.
Thats a basic tune up best buy will charge you $200 for. Yeah there are a few more things but really, this is what makes the most significant difference.
If something goes wrong and something breaks, then you have a system restore point you made before changing the settings to go back to how the system was. Pretty much no risk.