Thanks for the responses!
I'm happy to provide more details below.
@ mafist:I am using avast! 5.0 Free Antivirus currently.
Sorry, but I don't feel like testing each and every shield one by one. At least, not right now.
All I know is that it is "AvastSvc.exe" causing the issue with immense C.P.U. usage. "AvastUI.exe" is taking little to no C.P.U. usage at all.
@ scythe944:I first installed avast! 5.0 Pro over avast! 4.8 Professional Edition. (avast! 4.8 had an activated license, by the way.) The install of 5.0 went through without a single issue, so it seemed. (After installation I did not restart my computer, as I was not prompted to do so by avast!.) However, avast! 5.0 Pro itself, after installation, caused all sorts of problems. I had to use Revo Uninstaller's Advanced uninstall mode to remove both avast! 4.8 Professional Edition (Yep, avast! 5.0 Pro's install process did not remove any avast! 4.8 Professional Edition files.) and avast! 5.0 Pro, including their registry keys and left over files. Afterwards, I restarted the operating system. I'd also like to add here that transferring the existing 4.8 avast! license over to 5.0 was not as easy as
insinuated on this page. The "Your current remaining license period will be maintained. Simply download and install the new version." text on that page is blatantly not true. Rather, a manual copy of the current 4.8 license was actually needed, which I simply could not get to copy over to, nor work, in 5.0.
After restarting, I double checked that the Program Files folder didn't contain any avast! files, and sure enough, all that was left was an empty Alwil Software folder. Being a more than satisfied avast! 4.8 user for at least 2 years, I decided not to leave Alwil Software as I was previously a very happy user. I thought: 'Okay, so the avast! 5.0 Pro upgrade did not go well at all, but perhaps the free version will work. Why not give it a go?' So, I proceeded to download it. It installed successfully without any problems. This time, I restarted my operating system. Everything was running smoothly, finally.
Until now, that is. I don't know why, but browsing Windows Explorer is very slow. Mozilla Firefox 3.5 is now also going into Not Responding mode three times more often than it ever used to. In addition to this, when I played Grand Theft Auto IV today, launching the game was painfully slower. All of these slow downs, lags, and performance issues were not present at all in avast! 4.8. Period.
As far as conflicts are concerned, I really don't think this is a potential cause. I use only Windows Firewall, which has avast! 5.0 Free Antivirus permitted to fully access the internet. Windows Defender is fully disabled also. There is no other security software on my system, other than Blue Coat K9 Web Protection, a website filtering application, which never conflicted with avast! 4.8. Ever.
avast! 5.0 Free Antivirus is not reporting any issues or problems in its interface.
If you require any more details/information, I'd be quite happy to assist.
Oh, and one more thing. Here's a screenshot of avast! 5.0 Free Antivirus taking up the majority of the C.P.U. when trying to browse my Documents folder.
Using avast! 5.0 Free Antivirus was the first time in a long time I've ever seen Windows Explorer's green loading bar show up on such a regular basis.