The idea behind TETENAL's question is that if they are all PowerPC-only applications, then possibly Rosetta has locked up. Rosetta is the layer within OS X that translates all PowerPC-only programs into the Intel processors' language.
The only way to sensibly kill and restart the Rosetta environment is to reboot the computer.
Make sure you are using the latest version of Mac OS X (currently 10.4.8), as Apple have recently added great performance and stability enhancements to Rosetta, especially in 10.4.8.