How long have you had your computer?
What version of ESET are you running? I've noticed some
reports of slowed system performance with the newest (Beta) version ... not sure if that's relevant in your case.
My first instinct is that some kind of spyware/adware/malware has found it's way onto your system. You could try
CCleaner,
Spybot Search & Destroy,
Lavasoft Ad-Aware, or a similar program to detect and remove unwanted junk from your system.
I also wonder what you have running in the background. Sometimes certain programs (often unnecessarily) run during start-up and just sit in the background taking up resources without you knowing. This usually isn't a problem with 4GB of RAM, but you never know. Press
Ctrl + Shift + Esc to get to the task manager. Click on the "Processes" tab. Look for entries with higher-than-expected CPU and/or Memory usage. Those
could be your culprit(s).
To view and/or remove programs from startup, follow the steps outlined in
this article. (Scroll about half-way down the page to
Option 2; either
Method 1 or
Method 2).
There could also be a software or hardware driver conflict, but I would exhaust the above options first.