First, thank you so much for your quick reply!
Now, I have some issues still:
1) no, even when I close ALL apps, it still gives me the rainbow with the same frequency.
2) via Applications > Utilities > Activity Monitor, and click on the "System Memory" tab, I get this:

The 900 MB something FREE doesn't sound 'healthy' to me, but what IS healthy? I have no knowledge of this, sorry! 😟
3) via "the Finder, choose the "Go > Computer" option. Click ONCE on your main hard drive, and then go to the File > Get Info menu option in the menu bar" I get this:

So still 665GB available... this is ok, right?
4) I followed the steps precicely of the link http://guides.macrumors.com/Repairing_Permissions, and still no result... 😟
5) I verified the disk, and got this:

It seems as if I need to repair it, but can I just do it like it says "restart the computer holding down the cmd + R"? 😕
Now, I have an extra info to give, which might be of use:
After having this MacBook Pro for about 2 months only, I had an internal HD problem. It just stoped and it was replaced at the Apple Store in Amsterdam. Before that I had never had this problem, but after that it took about 4 months to start this problem!
What do I do now?? 😟
It's really bothering me, as I work with photography and design, and can't stand having to wait for my super machine to 'think'! 😠