Windows XP over two monitors, but one of them switches off at boot... how to fix? How to switch back to one monitor?

Windows XP over two monitors, but one of them switches off at boot... how to fix? How to switch back to one monitor?

When booting into XP (x64, Athlon II X2 245, 4GB RAM), my main monitor (got two 19" TFTs hooked up, two gfx cards, a 4650 (1GB, the primary monitor's on this) and a 4350 (512MB)) switches off. Logging in blind (cursor down key, typing password) gets me one screen, the secondary.