Can't email xls file as attachment

I sent an xls file to my sister and when she tried to open it Excel opened but she got this:

"(File).xls cannot be accessed. The file may be read-only, or you may be trying to access a read-only location. Or, the server the document is stored on may not be responding."

When she sent it back I got the same thing. If I go to sent mail then I can open it from there. I am using Excel 2000 and Outlook 2000. Maybe this is an Outlook question. Thanks to everyone in advance.
Mike

Hi there,

while opening ur attached file,Please do the following may be solution for ur problem,

after open the attached file it looks like the [read-only],but after opening u save as different name inur local drive..


this could be the solution for ur problem.

Thank you. Unfortunately saving it with a different name did not work.

I can answer my own question now. The problem is my internet filter, Cybersitter. The only way to make it work is to turn off Cybersitter temporatily.