Here is hwo I tackle this kind of complaint (sluggishness)..
Almost always it is one of three things:
1. malware/adware/spyware infection 80% of the time
2. file system maintenance issue: no disk space, excess temp files 10%
3. Resource utilization issue: XP with 512mb Ram & celeron, other background process dominating cpu utilization (antivirus, backupjob, indexing, etc..)
How I tackle it:
I have bootable DVD, CDRW, and also USB stick with drive image software & PC utlities and use whichever one seems appropriate.
1. I boot from my disk and clone the drive to external portable drive in case it all goes to crap... I have a good backup and can put it back the way it was before I showed up.
2. Boot up normally to check condition: PC make/model, BIOS version, OS version, what service pack/updates, what antivirus, drive configuration (partitions, # of drives, capacity), amount of total ram, amount of free disk space, what process loading on startup, amount of available ram after all startup process loaded.
3. Then I run utilities from my disk/dvd... in this order: ccleaner, hijackthis, malwarebytes, superantispyware, hijackthis again.
That will fix about 75-80% of the issues caused by malware.
If not, then you have to determine what is the cause of the problem.. identify the specific malware you are dealing with (see hijackthis log, running processes)... you may have to manually remove files and edit specific registry entries.
If the problem is ram/processes then I backup HKLM/LocalMachine/Software/Microsoft/Windows/CurrentVersion/Run and start deleting entries I feel like are expendable. You may want to use the msconfig utility rather than editing the registry directly. Shut off startup processes and un-necessary services (like wireless zero config for a desktop using wired ethernet) A bare-bones XP will take between 80-100mb. Most "bad loads" are in the 350-400mb range. On a 512mb machine that does not leave you much overhead to run any apps. Especially browsers.
I can't count how many users I have run into complaining about speed problems that were on an XP box with 256-512mb of ram and after booting up they were using 400+mb just to load XP & startup processes. Add RAM to resolve this issue.
In a worst case scenario you can reimage the drive.... but that CAN have some drawbacks:
1. After restoring from factory image you have to download and install endless MS updates, service packs, hotfixes, and reboots.
2. You will wipe out all the programs you have installed that you may or may not have installation disks for.
3. You will THINK you have a good backup, but then find out it is corrupt or otherwise unuseable. Or, you will THINK you have backed up what you need (docs, pictures, favorites) but forget your outlook PST & PAB files, or the Apple/iTunes folder.
4. You can't remember your pop3 email (or any other) cached password.
5. After spending 5-6 hours re-imaging, updating, reinstalling, reconfiguring you then restore your files and reinfect your new system too! Yep... seen it!
So, my advice (in summary..)
1. make sure you have adequate ram/diskspace
2. run ccleaner, mbam, and sas
If still bad, it may be easier to back up and reinstall OS... it is tedious but you will be guaranteed a good clean system.
You usually can re-image from the factory partition by using a keystroke (like F9, F2) at the BIOS splash screen... the same place where you hit DEL to enter BIOS setup.
You can look at disk configuration in compmgmt.msc and look at the HDD to see if you have a factory partition on your HDD.