Freezing At Log On Screen?

I have been using Win 7 Ultimate 64bit for about 6 months. About 2 months ago this behaviour started and I was unable to associate it with any changes I had made to the machine.Windows boot proceeds normally until the jingle and appearance of the logging on screen. At this point the computer becomes unresponsive to keyboard or mouse for a random time varying between 2-3 seconds and indefinitely. In the case of indefinitely I have to shut down by the power switch 3 second press and try again. My guess is that something I installed or something that installed itself is trying to call home at this time and prevents all other actions while it waits for a reply. Sometimes I have found that I can log in immediately after removing the network cable. I run Norton Internet Security which is up to date and reports nothing. If I try using the windows repair facility from the Win 7 disk to detect boot problems it reports that the boot that I had to abort was in fact successful. I guess it doesn't count anything after the log in screen.

The only time I ever got infected in the last 10 years was by a driveby trojan and I had to solve that by a drive image restore. That was a previous machine though.

I could solve this problem by restoring one of several drive images that I have but they are quite old and would have to reinstall quite a bit of software agin so I would rather not.All I am looking for is an opinion as to whether I have a windows problem or a malware problem. I never open attachments so a driveby hit is the only likely source of any malware.