See if MBAM will find any malware that is maybe the cause of freezes:

Malwarebytes Antimalware (MBAM) http://www.download.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html
You can also download it from http://www.gt500.org/malwarebytes/mbam.jsp

Install MBAM then allow it to update and launch after installation.
When MBAM opens, select the "Settings" tab then put a checkmark for "Terminate Internet Explorer during removal"
Proceed in running a quick scan by selecting Scanner tab

+++++ If MBAM will not install, please rename the installer mbam-setup.exe. Example: newtool.exe
Proceed installing the renamed installer of MBAM.

+++++ If MBAM will not run, go to the program directory of MBAM (e.g. C:\Program FIles\Malwarebytes Antimalware\) then rename mbam.exe to newtool.exe, double click newtool.exe to proceed in running a quick scan.

+++++ If you can't update MBAM, manually download the database installer from http://malwarebytes.gt500.org/mbam-rules.exe
See also: http://malwarebytes.gt500.org/database.jsp

+++++ If you need to re-install MBAM but encounter issue in re-installing, try using the MBAM Cleanup Utility by downloading it from http://www.malwarebytes.org/mbam-clean.exe

NOTE: If any of it found infection, allow it to fix the computer. Reboot after the removal process. Re-scan the system until no more infection can be found.
Please scan in Normal mode, scan only using SAFE mode if the PC shutdown during scanning and/or removal process.

If no infection found, please check the Event Log viewer in XP for any system or application.
To bring up Event Viewer: Start>Run then type eventvwr.msc
Hit OK. Select "System" at the left pane. Review the logs at the right pane. Note the error logs (event ID, source, and details of the log) that you suspect as related to the freezes.
Do the same for "Application" logs.