Excel locks up computer

This is for a client who just got upgraded from NT to W2K. He has a Dell GX1 with 128 MB and Office 2000. When he is working in Excel, primarily copying cells, his computer locks up completely, no Task Manager available. I have installed W2K SP 1 and 2, and also 1 and 2 for Office 2000. I have also completely removed and reinstalled Office 2000 and service packs. I am at a total loss. Could this be a hardware problem?