Slow as hell MacBook Pro! Help!

My 2011 MacBook Pro (4GB RAM, 320GB HD, Sandy Bridge i5) has gotten mind-numbingly slow. It's been like this for months and I feel it's just getting slower and slower.

Basically what happens is that it takes a good 30 seconds for Safari to fully open. It takes like 15 seconds for VLC to open etc etc. If I have Safari opened and I want to open Chrome? Forget about it! I don't have excess apps installed. The actively running apps at all time are Mail, Twitter, Menu Meters (network and CPU usage in status bar)

I've checked the activity monitor. CPU use even during this crawl is minimal. RAM can be 1GB free or 100MB free; it's slow as f*** regardless. If it were Windows I'd run a couple of registry cleaners and stuff like that. I've no idea how to diagnose this on OS X. I know laptop hard disks are slow but I have an old Dell Inspiron that costed $450 and it's faster at these mundane tasks.