I think that is not a Winamp problem. I'm sure you have a nVidia card and a driver version above 77.xx installed. There is a bug in these drivers. Try this:
• GeForce 6800/6800GT/6800 Ultra, Windows XP, Windows XP
Professional x64: NVIDIA Overlay gamma controls under Standard
mode do not work properly, resulting in overly dark or overly bright
To work around this issue:
1 From the NVIDIA control panel, click the Color Correction menu tree
2 From the Color Correction page, click the Apply color changes to list
arrow and select All, then click Restore Defaults.
3 Click Apply.