How to monitor Windows XP disk I/O?

I would recommend Process Explorer. It is replacement for Task Manager and among many features it has tracking of IO statistics. That should enable you to find which program is problem.

Once you know which program is to blame and if you need more detail, you can track which exact files are problem via Process Monitor (filtered to exact program since otherwise there will be too much data).

The Windows' Performance Monitor seems to do fit the bill. The program can be started from Control Panel | Administrative Tools | Performance or by running %SystemRoot%\system32\perfmon.msc /s. It probably needs elevated user rights to operate properly, as it's location suggests.

Within the Performance tool you can set various criteria to monitor, including relative, average or absolute disk read/write times, data amounts, queue lengths and a lot more - as totals or per selected logical disk.