This is a great article on how to tune Windows 7 for VDI with Group Policy.

 Rather than manually hacking Windows into some unsupportable format, I decided to try using the in the box features of Windows and did everything with a single GPO.  The net effect was that I was able to trim over 100MB of RAM from my VM and reduce the number of running processes by 6. 

You may be thinking to yourself, big deal, that's only 100MB and my new Dell R710 or HP DL380 has 96GB of RAM so why would I care?  Well if you can trim 100MB of allocated memory from 75 Windows 7 VMs, perhaps you can take that 7500MB of RAM and free CPU cycles and squeeze another handful of VMs on each host.

 Below is a screenshot of my two running VMs.  Each is Windows 7 Ultimate on Hyper-V 2008 R2 SP1.  I have Dynamic Memory enabled and limited my trimmed-down VM to 320MB of minimum RAM and the out of the box Win7 VM set to start at 512MB.  I originally tried to start at 256MB of each by my skinny VM would barely operate and the out of the box one wouldn't work at all.  These were the minimum settings I found where the machines would operate without throwing error messages about being starved for RAM.  I set the maximum RAM for each to 2048MB.

Source: http://communities.quest.com/community/vworkspace/blog/2010/12/15/tuning-windows-7-for-vdi-with-group-policy