Sorry Liz... I thought you might have been a *bit* techy...
I mean how many colours is your desktop set to use?
Right-click the desktop and select properties, then settings. It should say how many colours are in use. If it's lower than "Hi colour (16bit)" or "65526 colours" then SOMETIMES the dithering technique will cause colours to look darker/lighter than they should be.
If you need to change the video settings then make sure you read the popup message about clicking ok within 15 seconds of u want to keep the new setting. If the screen just goes funny, then leave it - after 15 secs it will return to how it was.