How much space is left on the drive--should be at least 20% of the total. How much memory do you have--you should max out memory. According to Mactracker max memory for your machine is 8 GB. I would also boot into the system disk and select utilities/disk repair. Run disk repair until it reports no trouble found. Ralph

How many applications are running in the background?

A common problem from exWinDUHows users is that they think closing the window quits all programs.

The red button kills some apps (System Preferences, iPhoto, Disk Utility, Calculator) but not others (iTunes, Web Browsers, eMail clients)

As a general rule, document-centric applications, those which create/use multiple windows, remain open. Single window applications will quit. To check, watch your menu bar when you close that window. If the application title changes you've quit the program.

Also there's a "throbber" or an icon in the dock under open applications.

If you have many applications running in the background and low RAM your machine will slow.

N.

Is the boot up process also slow? If so go to the disk utility and verify / repair the disk. I had this problem with my iMac (mine ended up being the hard drive going bad, but I was able to back up before it went completely out)

Seems to me your hard drive is at the end of its life time.

I now experienced the same and have to replace the HD.

Apple built in drive CPR (I'am not familiar with the English name for this application) advised to reformat the hard drive, but this appeared already impossible.