What are the 2 or 3 most useful ways (scans, defrag, etc) to keep machine from getting bogged down. I have some part of McAfee installed.

Anti-virus - AVG (www.avg.com)

Defrag your hard drive. Use the built in XP defragmenter (dfrg.msc). It's the only one that properly optimizes your system's prefetching to speed up boot time.

Go to Start->Run... and type or paste "Rundll32.exe advapi32.dll,ProcessIdleTasks". This will start a built in optimization routine that runs in the System Idle Process and shows up as "rundll32.exe" in task manager. It normally runs automatically every 3 days. It will run for 15 minutes to half an hour, so don't try to do anything too intensive while its running. If you get tired of it running, you should probably log off or restart to be safe, rather than ending the process.

Delete anything that you don't need in the Start->Programs->Startup folder, and in the Registry under HKLM->Software->Microsoft->Windows->Run and HKCU->Software->Microsoft->Windows->Run.

Go to Start->Control Panel->Administrative Tools->Services or go to Start->Run... and type "services.msc" to display services running on your machine. About 1/3 should be automatic, 1/3 should be manual, and 1/3 should be disabled, In my opinion. If you don't know which to change, search Google for "XP services" and read one of the many webpages that describe each service.