Will you please give the model # (extended model number) of the Dell Latitude? Some had 2 slots that would only hold 256mb each which adds up to 512mb total. You could possibly already be maxed out on the memory you can add. No way of knowing without a model # or physically accessing the laptop.
Are you using Windows XP? Or Windows 2000? What is your processor speed? Go to control panel -> system to find this.
Follow these instructions to view which programs are starting at boot. Will you please expand the command line and post an image of the programs located under the startup tab? At this point, please do not make any changes to any settings. ->
http://www.howtogeek.com/howto/windows/quickly-remove-items-from-windows-xp-startup/I think the first thing that should be attempted is to reduce resource usage as much as possible and scan the system for integrity.
Honestly, this is no power house setup and could possibly date back to around early 2000 (assuming this due to the Pentium 3 processor). It's difficult for me to know how slow, slow is. Whether you are maxed out on the memory you can add is yet to be determined. If it were me and memory could be added, I wouldn't. It would be best to save this money for a newer laptop with better specs. Maybe refurbished? Not my call though.
Will you please run AVG removal utility from here ->
http://singularlabs.com/uninstallers/security-software/This is to make certain any remnants are fully removed.