If you're looking to fix people's computers, the most common "problems" are a cluttered up hard drive, registry, bad system settings, viruses, spyware, etc. To clean up people's systems download CCleaner then load it up onto a flash drive and take it wherever you want. Viruses and spyware you just boot into safe mode and do scans. For system settings make sure their temporary file storage isn't set too high and that appropriate firewalls and passwords are set on the system and on the router if they have one.