Checking whats running
One of the tools that I have found very useful when it comes to troubleshooting my Windows XP systems is What's Running.
This tool has a familiar interface similar to Windows Explorer and you can check processes, services, modules, IP-connections and also drivers.
Its also possible to use the tool to control your system by starting and stopping services and processes and also easy to configure startup programs.
The feature I like most is taking a snapshot of the system and saving it for later comparison especially when something is not working properly. So you can compare a snapshot when everything is working fine to a snapshot when there is a problem and that comparison may shed some light on what the problem is.