Well, I also use Avast, the free version with the default anti-virus settings (no aditional features enabled) and I’ve no connection problem.
I run W7 Ultimate 64-bit on my desktop and W7 Ultimate 32-bit on my laptop. The FP2 was recognized automatically and works well in every USB mode, although Windows didn’t find all drivers.
I’ve done a quick web search and there seem to be multiple and different problems with Windows 7 (especially with the 64-bit versions) and MTP, some caused by anti-virus programs, some not. I would suggest you to do a web search and try the different solutions provided on other pages.