Minimum system requirements to Run Windows XP Home:
http://www.microsoft.com/windowsxp/h...n/sysreqs.mspx
You'll see the PC is within specification. for Windoze
You don't say which version of Office is being run, or what is actually being done on office. Equally you don't say if other applications are also running.
I'd go with Zinglebarb's observation. Boost the RAM if you can, ideally to a minimum of 256MB. On board graphics alone will be using some of the system RAM perhaps reducing levels to something unsustainable.
Will be worth doing a defrag on the HD too. If the HD is low on free space, that restricts the PCs ability to operate smoothly.
edit: I've amended the thread title slightly to something a bit more descriptive.