It looks as thought a setting has changed in your Start Menu settings. To change it back follow these instructions:
XP Start Menu:
1. Right click the Start button
2. Select Properties
3. Select the Start menu tab
4. Choose the Customize button
5. Select the Advanced tab
6. Scroll down Start menu items box and place a checkmark in the entry titled: Enable dragging and dropping
7. Click OK, then OK once again
Classic Start Menu:
1. Right click the Start button
2. Select Properties
3. Click the Start Menu tab
4. Select the Customize button
5. Scroll down Start menu items box and place a checkmark in the entry titled: Enable dragging and dropping
6. Click OK, then click OK once again.