I don't play Modern Warfare 2 - but some reports indicate that it has a virus in it.
What seems to be happening is that Avira is coming up with a generic detection of TR/Crypt.XPACK.Gen on a temporary file (perhaps ~B8.tmp) in C:\Documents and Settings\%USERNAME%\Local Settings\Temp.
However, "TR/Crypt.XPACK.Gen" is a generic detection - Avira is scanning the file and determining that it might be suspicious because it has been compressed with a commercial packer (a bit like a ZIP file). It is almost definitely a false positive that will be fixed quite soon.
If you like, you can head to the Avira Support Forums although where there is a short thread about it.