1) Go download, install and run this program (Advanced Windows Care, free):
http://www.iobit.com/advancedwindowscareper.html?Str=downloadAfter you run it, take a look at what it wants to change before you tell it to go ahead - for example, if you have it do the complete check, it will also check what items are starting up at boot - and it's default seems to be to remove - so make sure you put anything into the "ignore" list that you want it to continue to run at startup (such as AVG, firewall, whatever)
There's a RAM tool built in (look in Tools section) that might be helpful too - it periodically frees up RAM - but you'll have to decide if it's worth keeping it running in the background (I don't)
There's also a defrag tool at the same site, worth picking up (Smart Defrag)
2) Go get RegVac (free trial) and run the Scan/Cleaner. Use the System Optimize tool, and then do the Registry Pack.
http://www.regvac.com/3) Look at your options in any programs that run in the background, such as AVG (in AVG8 = Tools > Advanced Settings) In particular the "Resident Shield" is the one that does the continuous monitoring, so look at the settings - eg, is it scanning "all" files or just "infectable" files? - that sort of thing. Comodo Firewall/Defense has similar options to adjust - you can control how strong the analysis is, but tradeoff is time/system resources. Also check which of your programs might be set to either auto update or auto scan, because maybe that's what they're doing when your system slows down (I set all mine to manual, but you have to remember to do it)
4) Go to Black Viper's site and read enough to get familiar with what he's talking about regarding Services - this is a big area where system resources are squandered, and where security holes exist - so it's really worth learning about what the services are and which should or shouldn't be running. He provides some really helpful files (links below) but make sure you understand what they are and how to use them before applying them. I'm providing the direct links to the file downloads because sometimes they are hard to find on his site - so make sure you know what they are first by reading his site.
http://www.blackviper.com/WinXP/Archive/servicecfg.htmhttp://www.blackviper.com/WinXP/supertweaks.htmhttp://www.blackviper.com/WinXP/customREG_Home.phpThe only exception I take to his suggestions is the one about disabling System Restore. I won't do it because it's saved my butt on more than one occassion.
Some BV download links:
WinXPHome "Safe" Services Configuration file
http://www.blackviper.com/downloads/WinXP/Archive/!Safe_XP_Home_SP2_Start_v203.zipWinXP(all) BlackViper Annotated Comments in Services Description
http://www.blackviper.com/downloads/WinXP/Archive/!BV_XP_SP2_Descriptions_Values_v201.zipHome SP2 Safe Start
http://www.blackviper.com/downloads/WinXP/Archive/!Safe_XP_Home_SP2_Start_v203.zipPro SP2 Safe Start
http://www.blackviper.com/downloads/WinXP/Archive/!Safe_XP_Pro_SP2_Start_v203.zipHome SP3 Default Start
http://www.blackviper.com/downloads/WinXP/Registry_Files/!Default_XP_Home_SP3_Start_v300.zipPro SP3 Default Start
http://www.blackviper.com/downloads/WinXP/Registry_Files/!Default_XP_Pro_SP3_Start_v300.zipXP Services Descriptions - Default
http://www.blackviper.com/downloads/WinXP/Archive/!Default_XP_SP2_Descriptions_Values_v201.zipXP Services Descriptions - BV annotated
http://www.blackviper.com/downloads/WinXP/Archive/!BV_XP_SP2_Descriptions_Values_v201.zip