For the full scoop on the Y2K readiness of Microsoft Office Professional for Windows, visit this web site:
http://computingcentral.msn.com/.

For information on Microsoft Office Standard, visit this site: http://computingcentral.msn.com/guide/year2000/msy2k/productinfo/office.asp

In summary:

Office 2000 Professional is Y2K compliant.
Office 2000 Standard is Y2K compliant.

Both Office 97 Professional and Office 97 Standard need the Office 97 Service Release 2 to be Y2K compliant. Get it here:
http://officeupdate.microsoft.com/articles/sr2fact.htm

Both Office 95 Professional and Office 95 Standard need the Office 95 Year 2000 Update. Office Get it here:
http://officeupdate.microsoft.com/articles/O95y2kfactsheet.htm