I do an every three month deal. It works like this: If a customer calls me and need what I call my basic cleanup/speedup package, I charge $30 plus cost of gas. If they are under my maintenance contract I'll go out every 3 months for a flat $25. This may not seem like much but It does not take that long most times, depending of course on the speed of the system about 1-1.5 hours. Here is a list of whats included.
Spyware/adware/mal-ware/virus scan ( using spybot s&d, ad-aware, and McaFee Stinger )
delete all temp, old , junk files and internet garbage.
remove unused programs completely
Defragment using jkdefrag also defrag the pagefile
Make sure windows is up to date
Update antivirus software
clean broken or junk reg files
basicly make sure everything is running smoothly
Some may say this is undercharging but this price does not include any data back up, installing new software, tweaking, or anything else. This is strictly maintainace/tune up. Also I try to work it around going to another customers house.