What are your memory and multiprocessing settings?
> The OPENGL feature of the graphics card was first not seen by After Effects, thats why I installed the newest drivers from NVIDIA.
Don't use OpenGL for rendering unles you're just rendering a rough preview.
2 Processors, 0.75 GB per processor, 2.5 GB for whole suite.
Deactivated OpenGL still slow.
I'm afraid your posts so far are of no use to anyone. You did not provide any info on what footages and effects are involved, thus blanket statements along the lines of "it's getting slower and I'm aggravated" are pretty pointless. The behavior could be perfectly normal if e.g. a memory intense effect keeps clogging up your RAM or a temporal effect keeps accumulating buffers over time...
Why are you only using 2.5GB of your 8GB?
Tell us about the size of your composition frame (i.e., how large your movie is) and what the sizes of your footage items are. If, for example, you have a large footage item used in your composition, then 0.75GB per process might not be enough, so the background rendering processes are going to shut down and all rendering will be done by a single process. See this post for more about memory settings.
Tell us about what kinds of footage you're using, too. It may be that the decoder used for one or more of your footage items is the bottleneck.
Also, have you installed the After Effects CS5 (10.0.1) update?
It seems that the AE project I got from someone else just had to many items to render. After 27 hours it was finished...on a faster machine with 16 GB of RAM and a quadcore with 3 Mega per proc it still rendered slow.. for me After Effects is unfortunately not the right kind of software.