Regarding the crashes in tdx.sys:
This is actually a known problem (a bug, really) in Windows 7 beta (build 7000).
I spoke with some people in Redmond last week and they confirmed the bug is in the tdx.sys driver itself and has already been fixed. We could confirm that with the latest internal Win7 build (7021 IIRC) the problem is indeed gone.
Now, you probably don't have access to these interim builds so I'm afraid you'll have to wait till the RC version (there aren't going to be any more public beta builds AFAIK).
Alternatively, there's a work around that you can use: uninstall the avast Network Shield, Web Shield and Internet Mail providers (i.e. all network-related stuff). This will leave avast only with the basic on-access functionality (file system scanning only) but in reality, this will suffice to block the vast majority of malware.
To do this, open Control Panel, click Programs and Features -> Avast! antivirus -> Change/Remove -> Change and untick the three boxes (Network Shield, Web Shield and Internet Mail).
Hope this helps,
Vlk