Do not run any such "cleanup" programs. The only guarantee is that they will cause problems and frustration and are likely to cost you time and money to fix.
If it is only Web browsing that seems to be slower than it used to, that is often the result of such sites becoming ever more demanding of hardware and bandwidth. Short of paying for faster Internet service, buying additional memory, or a faster Mac, there is little you can do to cope with such so-called progress.
If you are experiencing general slowdowns or "freezes" unrelated to Internet activity, the problem may be related to your computer or the software you installed and is something you may be able to fix, or at least improve. Aside from hardware failures and software that you install, including OS upgrades, there is no reason that your computer should not work precisely the same as it did when it was new.
Without a more specific description it is difficult to provide specific guidance. Read the following to determine if something may apply to your concern:
General Mac maintenance: Tips to keep your Mac in top form
General purpose Mac troubleshooting guide: Isolating issues in Mac OS X
Creating a temporary account to isolate user-specific problems: Isolating an issue by using another user account
Identifying resource hogs and other tips: Runaway applications can shorten battery runtime
To identify potential hardware problems: Apple Hardware Test
To resolve startup issues and perform disk maintenance, use Disk Utility.
Safe Mode or "Safe Boot" is a troubleshooting mode that bypasses all third party system extensions and loads only required system components.
Read about it: Starting up in Safe Mode
To repair a potentially corrupt hard disk, so that you may recover its data prior to replacing it, and subsequently reinstall Mac OS X: OS X Recovery (applies only to Lion and later versions of OS X)
What is the year, CPU speed of your iMac?
2 GBs of RAM is barely enough RAM to run just OS X 10.8 Mountain Lion, alone.
Run additonal applications with OS X and your iMac is going to slow down in performance.
Every iteration of OS X, especially Intel versions uses more and more CPU, GPU, RAM memory and hard drive resources.
Without knowing more about your iMac, we can't advise you about how much additonal RAM you need to add to your iMac to help with its performance.