Cannot Display Video Mode

Hi all, I am a newbie to this forum and I am encountering a problem that is posted and discussed several times on this forum: "Cannot Display This Video Mode" after starting my Dell Desktop PC. I am getting this after reinstalling Windows XP. I got a Dell Dimension 4550, OS XP AV05 BIOS with a 15" LCD  and an ATI Radeon 9700 Pro graphic card. I read multiple posts about the subject and somehow the tip for shutting down the monitor to synchronize the signal with the ATI card does not bring any solution. The other solution of starting in XP safe mode wont work either, somehow safe mode wont start on the PC, hitting F8 multiple times wont bring the menu to select the safe mode, it just says "Cannot Display This Video Mode". The screen did work before reinstalling XP. Can anyone give me a hint or an idea? Thanks in advance!