How can I open this attachment?

Clarifications

It could be that you inadvertently placed a copy of the file into a different folder
other than the original it was stored in.

you might have to get office to repair itself. there are a couple of ways to do it. but one of them is via add/remove (or in win7 program features) via the control panel.

when you click on the office line item, it will give you a chance to repair the program, like in win7.

for other versions, you may have to click uninstall in order for the office installation menu to pop with the options to repair, modify or uninstall.

Can you save the file to your desktop or documents folder? Try saving it on the desktop with a new name. If this works, then open Excel and navigate to the desktop and try opening the file from within Excel.

there are settings that prevent downloading / opening potentially unsafe files
but I don't remember offhand where they are located exactly

Step 1: Locate the folder
The folder location is stored in the registry in the following key;

Outlook 97

HKEY_CURRENT_USER\Software\Microsoft\Office\8.0\Outlook\Security

Outlook 98

HKEY_CURRENT_USER\Software\Microsoft\Office\8.5\Outlook\Security

Outlook 2000

HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Outlook\Security

Outlook 2002/XP

HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security

Outlook 2003

HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security

Outlook 2007

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security

Outlook 2010

HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security

Step 2: Get to the folder
1.Open the OutlookSecureTempFolder registry key from the location provided in Step 1.
2.Copy the path from the key.
3.Open Explorer
4.Paste the address in the Address Bar and press Enter
5. Delete the contents of the folder

What exactly is the name of the document? You say there's a date in it. If it's got slashes in it, it could stop it saving successfully.