Check if there is a lot of RAM! Consider to upgrade it!
If not, then there are probably a lot of hard disk read/write accesses. Check the hard disk for bad sectors (SeaTools, chkdsk c: /p). Attention: this may need your Windows XP install disk afterwards, but may increase the speed a lot!
Also each read/write-access on the hard drive may provoke the anti virus software to check each moved data for viruses on every file access. Additionally it may access the hard disk for virus signature information, too. Check for the CPU- and HDD-Usage and Page faults of your AV-Software (via taskmgr) and disable some less needed features of it.
Updating Antivirus-databases may take some CPU-time, too. Consider to replace the AV-Software some update-less Software like Panda Cloud Antivirus (especially if the computer is already always online).
Speaking of Updates. Consider manual updates instead of automatically started updates. Then they don't use the CPU if you wouldn't expect them to.
A large delay when right-clicking something might be caused by a cluttered registry. Use RegCleaner 4.3's auto-clean to remove the useless entries.
Also defragment your hard disk to improve HDD-perfomance. I use MyDefrag (it's opensource, easy-to-use, needs no configuration, defaults make a lot of sense).
When setting up Windows the next time, install it on FAT, not NTFS. This causes higher risk of file loss, but higher performance. Also, Windows is able to repair itself even on FAT most of the times. Install programs on a separate partition to reduce boot time just by defragmentation.