Strange goings on with Excel 97

I have a user who’s PC is running Windows NT 4.0 (SP4) with Office 97 and Outlook 98 and this problem has only come up in the last two weeks. When logged onto this PC as the user, Excel files cannot be opened with in Windows Explorer or if they are sent as e-mail attachments. The only way, is to open Excel then open the file. This does not happen in any other software package loaded onto the PC, only Excel. But, when logged on as the domain Administrator, it works perfectly. This does not appear to be a permissions issue and I have tried re-installing all the software again but with no luck. It seems to have lost a link somewhere, but I cannot find where. Any suggestions for an under paid, overworked, very stressed British I.S tech?