Grab a copy of adwcleaner.
Run a pass and see if you have some uninvited guest.
A couple weeks ago, I had to download an update (one of those Microsoft ones that keeps bugging you for days to restart to install it before eventually taking matters into its own hands). I've had problems since then with my browsers lagging or slowing to a snail's pace and dragging the rest of the computer with it, usually after a couple hours of working fine. This happens in both Opera and Firefox, both of which are up to date. It often happens if I'm using Netflix or youtube, but it also happens when I'm just on text-based websites. As soon as the browser starts lagging, I open the task manager and see that CPU usage has spiked. When I look at the resource monitor, it's either the browser or perfmon.exe that's eating up the bulk of it. When I close the broswer, the CPU usage immediately drops.
When this started happening, I didn't immediately think it was an issue because my computer dislikes updates, and it always takes a day or two to work out the kinks after doing one. Usually this manifests as getting high CPU usage for a day or two after, at which point I usually get irritated and system restore to an earlier time, which takes care of the problem.
I've tried to system restore this time, but the only restore points from before the update say that the backup drive cannot be found (I regularly back up files to an external HD but I've never created a restore point on a different drive, so I don't know where this is coming from). I also ran AV and malware scans just in case, but they both came up empty. I'm doing grad school online right now, and I can't have my browser quitting in the middle of tests. Any help would be much appreciated.
I'm running Windows 7 64-bit. I use AVG free for AV and malwarebytes free, though I switched to MSSE yesterday just to see if it helped at all.

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