HELP! 17" iMac 2.0core2duo

I've had this computer about a year and a half and i LOVE it! it has been very fast and very responsive. I upgraded the ram to 2gigs. Recently it has been slowing down *especially when trying to install or upgrade software* and i thought it was because i had the 160gb hard drive full. So i decided to back up my music and photos and go ahead and reformat and reinstall OS X. Well the install of OS X took over 8 HOURS! and updating little iphoto on software update took 20 minutes to verify and install....I'm just wondering if the hard drive is failing or if my computer is just jinxed. If the hard drive is failing where can i take it to have it replaced as i do not think im mechanically inclined enough to break open this beast
thanks in advance for all your help guys.

You can check the SMART status of your HDD. How full is your HDD? (Select HDD and use file>get info from menu bar)

What are the specs for your iMac? (processor speed)

Your processor is a core 2 duo, meaning you probably have 2 GHz or more. Plus the 2 gigs of RAM which should make things faster. Assuming that the 160gb drive isn't full, have you run disk utility and resetting safari? Disk utility might help alot and resetting safari should clear out little temp files.