Copy the c:\windows\system32\wpa.dbl file to a CDRW or floppy. Then put the Sony OS CD in the CD drive, reboot and get into BIOS settings, and make sure your CD drive is set ahead of the hard drive as a boot device. Save the settings and restart. The system should boot to the CD and let you restore the Sony-branded OS to it. Put your CD Key from the sticker on the machine or CD box in if needed.
When you finish re-installing the OS, copy the WPA.DBL file from the floppy back to c:\windows\system32, and you should avoid having to mess with activation.