Computer crashes upon closing Excel

[Mark]

Whenever I open a workbook I always get the EULA
confirmation window. Then, when I close the workbook,
Excel hangs up and stops responding. After Ctrl, Alt and
Del to exit, no other application works and I get the "Not
enough system resources to open..." error. I then have to
do a hard reboot.

Help! Any suggestions on causes and fixes?

Thanks.