You're right, Mark, about nothing wrong with the specs. It looks like you customized your HP ProBook 4530s, as the specs you provided don't fit with HP's pre-configured models.
I do know that avast! Antivirus should run just fine on your machine because I tested it with a virtual machine (running Windows 7 Ultimate) that was allocated two out of eight cores of an old Intel® Core™ i7 720QM and 2 GB of RAM. The virtual machine did not experience any of the symptoms you listed.
It is possible that you are having a hard drive failure.
Even when I have no applications open, simply closing and opening windows, lags. Or opening the start menu, or dragging icons across the desktop. sometimes even the cursor lags.
I confirmed this exact thing happens when I instructed a damaged HDD to read a file that was stored where the damaged sectors were.
The total disk activity fluctuates between %50 - %100
In my test, the HDD indicated that it was busy... that is, busy trying to read something it couldn't read. avast! may be trying to access files that are damaged, which makes the hard drive churn and seem very busy.
This can be confirmed by trying the suggestions in this very relevant answer on Super User.
If the problem is in fact a hard drive failure, back up your data!!! Then, check to see if the drive is covered under warranty. If so, get a new hard drive. If not, you should purchase a new hard drive or perhaps consider a solid-state drive, if one interests you.
If the problem turns out not to be a hard drive failure, consider the other answers here, as they are also possible explanations for why you are experiencing trouble with your laptop.