Has the laptop become slower over time? Most of the time a slow computer doesn't need a new CPU so much as it needs to be cleaned out (clean up or even reinstall Windows).
Changing the CPU on a laptop is a tricky and expensive process, and generally won't result in a huge upgrade. Your model has decent specs already, especially for Windows XP. A RAM upgrade is a lot easier but I'd do a few malware scans first, remove unneeded programs and files, etc.