Hi Art, welcome.
Presumably it's nothing obvious like having LR in the Startup folder?
Assuming not, do you have a memory card plugged in to your system or are you plugging one in after a restart (note that LR can think an external hard drive is a memory card)? There's a setting in LR to auto-import when a memory card is detected (which means LR will start and kick the import dialog into life), but on my Win7 system it does NOT cause LR to auto-start if I boot with a memory card already inserted. So am at a loss to explain your particular brand of problem.
Suggest for starters you check this setting: start LR and go to Edit, Preferences, Import Tab....first entry is "Show import dialog when a memory card is detected". If this is checked, try unchecking it and click on OK. Close LR, restart the PC to see if the problem is 'fixed'. If this works, OK we can start investigating a little further to see why it happens on a system restart...
If the entry is already unchecked then we need one of the LR gurus to step in....