Which file system are you using, FAT32 or NTFS ?
Have you converted from FAT32 to NTFS at any stage, and if you did did you ensure that the Allocation units have 4096 byte clusters?
Open a DOS window and type: chkdsk
You should see a reading pertaining to: 4096 bytes in each allocation unit.
Another reason for slowing down is spyware and Trojans, so you should use SpyBot SD, AdAware SE and SpywareBlaster to keep your system clean.
If you are using the NTFS file system then a fragmented Master File Table will also slow down the system.
Download the evaluation copy of Diskeeper 8 PROFESSIONAL (Not the Home version) and run a boot-time defragmentation to correct this problem.
It does NOT matter if you are using XP Home or Professional, but only Diskeepr Pro has the ability to defragment the MFT.
XP uses a slimmed down version of Diskeeper so installing this program will replace your existing defrag.
When you uninstall Diskeeper 8 your old defrag will be restored to use.
I consider that Diskeeper 8 Professional is well worth the money as it can be set to defragment all of your drives automatically.
Really Microsoft should have integrated the FULL version of Diskeeper because of its labour saving attributes and technical ability.