You never mentioned your client version so check the Avamar compatibility matrix to see if your current supports Windows 2008. If it doesn't then upgrade it to the latest client release in the following way..

1) Download the latest plugin_catalog.xml file
ftp://avamar_ftp:anonymous@ftp.avamar.com/software/v4.1.105.95-plugin_catalog.xml

2) Upload the file to the Avamar utility node and use it to replace the current version located at
/usr/local/avamar/lib/plugin_catalog.xml

The Avamar system will then be able to recognise clients newer than itself.

3) Make sure the new plugin_catalog.xml file has the same permissions as the old file... see below for example
-rw-r--r-- 1 root admin 391K Mar 20 13:58 /usr/local/avamar/lib/plugin_catalog.xml


4) Restart the MCS following the instructions in the Admin guide so the new .xml file is read.

5) You can then download the latest Windows clients and install them following the instructions in the v4.1 Backup Client guide
ftp://avamar_ftp:anonymous@ftp.avamar.com/software/AvamarClient-windows-x86-4.1.105-95.msi
ftp://avamar_ftp:anonymous@ftp.avamar.com/software/AvamarClient-windows-x86_64-4.1.105-95.msi