Is the workbook on the networked PC itself, or are you running it from
a server?

I've had a similar problem on our Novell network. The culprit in our
case is McAfee's VirusShield, which is their equivalent to the Norton
Antivirus AutoProtect. If VirusShield is on, we cannot work with Excel
files stored on the Novell server. But I'm not sure if VirusShield
causes trouble with macros on the local PC.

In short, turn off auto-protection and see if you can use the workbook.
If so, see if you can configure the AV software so it leaves your
workbook alone.

Otherwise, turn off non-essential processes in the Task List one-by-one
(HP printer software installations sometimes cause trouble) and see if
one of those is the problem.