is there such a piece of software that can stress test a webserver on high load for a couple of hours?
Or maybe an online service?
Or some software for my windows XP?

http://www.webhostingtalk.com/showthread.php?s=&threadid=236249&highlight=stress+apachebench

Just replied to this one yesterday :)

glad you posted that thedavid. i thought i was going mad :)

Also Microsoft has available the Web application stress tool: http://www.microsoft.com/downloads/details.aspx?FamilyID=e2c0585a-062a-439e-a67d-75a89aa36495&DisplayLang=en