2009 I-Mac running slow

Hello all. My I-Mac, which I believe to be the 2009 model, has recently been functioning very poorly. It began abruptly yesterday where one hour I was watching videos on youtube without an issue and then it became very unresponsive shortly after. It had become so slow that I performed a hard shutdown as the computer would not respond to my clicking on simply the apple logo.
After this shutdown, I assumed it would function properly, but then it persisted. Symptoms of this include the dock not responding immediately, whereas typically it would respond, when I dragged my mouse over the icons. Loading applications takes very long as does loading websites. Despite this, loading my homepage does not take very long, and according to my activity monitor, neither the RAM nor the processor are being strained to their limits.
The model has not been updated to Mavericks yet, but is on the 10.8 version. If anybody could help me with potential resolutions it would be greatly appreciated. I think it could be a hard drive failure due to the shutdown, but I find that unlikely as the symptoms were present prior to the shutdown. Alternatively, it may be I need to reinstall or upgrade the OS; i'm not sure, so perhaps that's where you all could start in diagnosing this problem.
Thank you whoever responds.

I would start by running some tests on the hard drive to see if it's OK. Also, how much free space is remaining on your hard drive? A drive that's fairly full will begin to slow everything down.

To check your hard drive: Boot to the recovery partition by holding down Command plus the r key when booting the machine. Once in recovery, select Utilities and then Disk Utility. Highlight your Macintosh HD on the left side and then select Verify on the right side.

slow

chscag said:

I would start by running some tests on the hard drive to see if it's OK. Also, how much free space is remaining on your hard drive? A drive that's fairly full will begin to slow everything down.

To check your hard drive: Boot to the recovery partition by holding down Command plus the r key when booting the machine. Once in recovery, select Utilities and then Disk Utility. Highlight your Macintosh HD on the left side and then select Verify on the right side.

Click to expand...

I agree with all of the above and in addition I would suggest also verifying and if necessary repairing permissions.
Total HD memory can certainly be a problem if you're running short, you need roughly 10% free. Background apps like time machine plus third party apps like dropbox and others can also chew up a bit of memory.
How much RAM do you have?

Thanks you guys I'll give it a shot and let you know. Space should not be an issue as I have about 500 GB free, haha. Also, I have 8 gigs of RAM. My computer also has had an encryption installed for about 6 months, but it has not been an issue up to this point.

Well I scanned the hard drive and it verified that everything is ok. I didn't check whether the OS X base system is ok though.