Hello and welcome to the Windows 7 Forums
I had a VERY similar problem on my Gateway notebook with Microsoft Security Essentials, but way more powerful hardware.
After MONTHS of trying to figure out what the problem was, I uninstalled Microsoft Security Essentials.. BAM! Problem solved.
It's great on resources but on some systems it seems to slow things down immensly at start-up.
I suggest you try that, and if it works, you'll have to make a switch to a different anti-virus just as I did, which is unfortunate because I loved Microsoft Security Essentials.