Help! 27" iMac bought In 2009 has never gone a day without fighting spinning wheel of death

Question

We bought this at the end of 2009, and ever since has had issues with getting the "beach ball" or spinning wheel of death anytime you click something new, try to open a file, or a program or any link, etc. It soins for about a minute then is good to go, but sometimes lasts longer and happens constantly! How can I fix this?! Its so annoying! I paid good money for this and my 6 year old MacBook Pro with Snow Leapord is blowing this one away! Here's the details:
Model Name:     iMac
Model Identifier:     iMac10,1
Processor Name:     Intel Core 2 Duo
Processor Speed:     3.06 GHz
Number Of Processors:     1
Total Number Of Cores:     2
L2 Cache:     3 MB
Memory:     4 GB
Bus Speed:     1.07 GHz
Boot ROM Version:     IM101.00CC.B00
SMC Version (system):     1.53f13
Serial Number (system):     W8******5PE
Hardware UUID:     0D367BAA-B1E9-537B-BEED-971531A63954
Then recently it did an update, restarted and is now asking for login credentials which we have never had set up...
PLEASE HELP!
Andrew
<Personal Information Edited by Host>

Answer 1

So you waited 4 years to report the issue, hard to have much compassion. I'd recommend by reading and using Activity Monitor to read System Memory and determine how much RAM is being used and also Spinning beach ball Troubleshooting. Finally when posting to a public forum DO NOT post private information such as serial numbers, addresses, phone numbers etc. I've asked the moderator to remove your SN info.