XP Pro diagnostic programs?

Hey all, not sure if this is the right place to post but here's the question.
I am getting into the PC tech repair side of things and wanted to know what professional diagnostic software is being used right now. I have seen that some sites mention using post cards to check XP systems etc. I have no clue what I should be looking for - either for software diagnostics and for certification books.........there's so much and I don't want to make a mistake in getting the wwrong stuff - its not cheap. Any help is appreciated. Again, sorry if this is in the wrong forum...............later

each windows component has its own diagnostic tool really, here are some general ones built into the o/s itself

perfmon.exe for performance issues

msinfo32.exe for general information summary and system overview (the tools menu drop down up on top actually has a few more tools in there once you open this program)

taskmgr.exe for monitoring system resource usage

msconfig.exe for services and startup management

administrative tools from control panel...use these if you need to troubleshoot something very specific

eventvwr.msc for viewing event reports, very helpful usually

and finally...the good old built in help functionality, when you go it from the start menu it also has a tools section with some cool stuff that report info about your machine in a more user friendly interface