One of the common problem for XP running slow (no high CPU) is disk defragmentation. If this is your case, defrag the hard drive.
For XP running slow with high CPU, you may have Indexing services running. If you do, turn it off permanently this way:
1. Double click My Computer.
2. Select View | Explorer Bar | Search.
3. Click Change Preferences.
4. Select Without Indexing Service.
5. Click No, do not enable Indexing Service.
6. Click OK.