Locked Out of Windows XP (activation problems)

Last night I shut down my PC computer. In the morning I turned on the system and Windows XP began to boot. It stopped on the log in screen. A Windows Product Activation window appeared: "This copy of windows must be activated with microsoft before you can log on. Do you want to activate windows now?" Yes or No.

My brother build this computer and activated windows a several weeks ago. I have been using the computer without this screen or any problems for awhile.

If you hit "No" - Nothing happens and the same login screen refreshes with the same question.

If you hit "Yes" - A screen with a lock and a key appears, saying, " Windows is alreadyactivated. Click okay to exit." Whether you click okay or click the x in the top right corner of the screen the same thing happens. You get kicked out to the same login screen.

Now I can't access anything. Not even safe mode. Maybe I am doing something wrong. Does anybody know how I can get into Windows so I can run some virus software? I haven't had the computer that long so it seems unlikely a virus has affected the system, but I'm can't be completely sure. I can not check anything because I can get into Windows. Is there another way to access the system? I can't even seem to be able to reinstall Windows. I changed the bios setting to boot the cd rom first but it doesn't seem to recognize the reinstallation cd.

Any suggestions for a next step. Any help would be greatly appreciated.