I'm still using AVGFree and haven't noticed any slowdown associated with it, though I'll keep a lookout
Mike Lin's startup contol is a great tool
http://www.mlin.net/StartupCPL.shtml
That in combination with checking which services are loading automatically is a good start
Right click My computer > Manage > Services Tab and look for running services. You can check which are set to start automatically, manually etc.
Tip before setting anything to disabled, check the dependencies and go to manual start if unsure as a first step.
Also keep notes as it's easy to lose track.
There are also some in there which are set to start automatically and IMHO are a security risk, Remote Registry is one in particular, though later service packs of XP have changed this default to disabled.
It's not uncommon to find services related to uninstalled programs and virus scanners still listed, these are safe to disable.
Whilst in the computer management, it's also a good idea to check the error logs, particularly System and look for errors as a slowdown which won't give popup mesages, like drive access will be listed.
Often times drive access errors will just need the cable reseated, but it alows you to catch drives before they fail.
I check the error logs weekly as good practice - like checking the oil, water and tyre pressures.
peace out
Glenn