With only 4GB of RAM, it's highly likely that memory is the issue. As Jim notes, check the page in/page outs in Activity Monitor and tell us what it says. Also let us know how big your hard drive is and how much free space is left on it. Also let us know how many apps you typically have open and what you see in terms of CPU usage etc. when you look at Activity Monitor. Be aware that Safari still has memory leaks - if you run it long enough without restarting it, and with enough windows open, it can eat up a LOT of memory.
0.1% with viruses, Jim? Even that seems high. There are a few trojans, "scareware," spyware and other assorted nasties out there, but almost all of them require the USER to
actually install them. A virus, by definition, replicates itself. There IS malware - but most of it requires the active participation of the user. The likelihood of this being the source of the problem is rather low. However, since you have asked about MacBooster, you haven't, by any chance, installed MacDefender, OSX/MacProtector, OSX/MacSecurity, Premium Opinion, pirated copies of iWork, iPhoto or Photoshop, MacSweeper...?
You do NOT need MacBooster. See discussion here - it neatly sums it all up:
https://discussions.apple.com/message/25257326#25257326