I have read now that this is not possible (http://www.thescripts.com/forum/thread27...).
Instead they propose that f.ex. in a C# asp.net site you run ressource heavy processes in a new thread with a low priority so that they don't consume the system from other "normal" processes.
I'm still wondering if it isn't possible at the level of the site or IIS itself to set a limit on how many ressources it may consume or a priority on the processes?

I'm not sure about IIS5 but in IIS6 you can completly limit processes as either CPU percentage limitization or memory usage limitization (both virtual and conventional separately) or both.
to do this you need to make a copy of the default virtual application pool for each site.
to do this, open IIS and expand your servername then application pools. you can right click on the default application pool, select all tasks and save the configuration to a file. then you can right click on the application pool folder and select new app. from file and use the file you just saved. Create the app pool with a different name and viola, a new app pool for your website.
now go into the properties for this application pool and you can set all the limits you want within the first two tabs (memory, cpu utilization etc.)
now go to your website's properties in IIS and change the application pool to the one you've just created and configured and hit OK. (apply this to all subdir as well if asked). that's it.

hmm, yes, I think that was what I thought was possible for IIS 5 as well. But apparently it isnt :(