MacBook Pro :: 2011 13" I5 Slowed Down, Increasing The Performance?

Machine Details:Macbook Pro 13inch model  (2011)/Core i5 4gb RAM 250gb Standard harddrive 

The laptop was running like a dream and much better than I remembered my last mac (Macintosh LC) ever did. Ive had minor hickups this last year (a trackpad that went bust under warranty and a broken power supply) otherwise its been a wonderfull experience. The final hiccup that might be a cause, not sure hence I will mention it is that my battery is Conditon: Replace Now for about a month now.  

The laptop, as always was left ON, on its own connected to the wifi home network, when I returned the computer was working very slow. Any load on the harddrive/CPU. The cursor starts lagging (jumping), any keypresses are registered with a 2-10second delay. Any vidoes that i would try to play would suffer freezes in framerates/jams but audio remained fine. First step i was afraid that i may have contracted one of the new trojans so i downloaded an antivirus program, scanned the drive, but came out clean. Second step was to format the drive and perform a fresh install of OSX but no luck. Being inexperienced in Apple products i assumed that my harddrive (used to PCs) was on the fritz and went out and bought a 160gb Western Digital Black series 2.5inch harddrive.

I placed in my installation OS 10.6 CD formatted the drive through the disk utility and performed a fresh install. Before doing any steps I went through the update cycle a couple of times till only the thunderbolt update remained (havent updated as I never used it). Sadly it hasnt solved the problems with cursor repsonse lag, video playback, and keyboard input lag under load. On this install I have performed:

Verify Disk Permissions Many were found so I performed Repair Disk Permissions
Verify Disk Came up with "Appears to be OK"
performed a Pram reset (CMD+option +P +R) 

My main problem is that I live in europe nowadays (Poland to be precise) and cant go to a genius bar as they dont exist, and sadly the support is a tad behind the US standards and any minor repair will be at least a 4 week wait?

Info:
MacBook Pro, Mac OS X (10.6.8)