The Microsoft Knowledge Base article,
Outlook Express message appears blank and has an ATT000XX.txt or an ATT000XX.htm attachment says to:
1. In Internet Explorer, on the Tools menu, click Internet Options.
2. On the General tab, click Delete Files, click OK, and then click OK again.
3. In Windows Explorer, click to expand the hard disk letter, click to expand Windows, and then click to expand Temp.
4. Select all of the files, and then on the File menu, click Delete.
5. Click Yes.
If that doesn't work, go to Start>Run, type "regsvr32 inetcomm.dll" (without the quotes), click OK. See if that fixes things.