No problem. When you boot to the first gray OS X installer DVD, you'll eventually get the standard menu bar at the top. As soon as it appears (you may have to click past a couple of other questions or screens first), launch Disk Utility. In the left pane, click on the far left physical hard drive icon, which should be your internal drive, assuming you have no external drives connected and turned on. Anything indented to the right underneath the physical drive is a logical partition on that drive. Like so:

With the physical drive highlighted, you'll have the Partition tab at the right. Click that. Where the drop down menu above the graphic representation of the drive says Current, change it to something else, even if the drive already is one large partition and you still only want one. Select 1 Partition from the menu. That will now allow you to click the Options button below the graphic. Make sure the GUID Partition Map radio button is selected and click OK.
All new partitions you create will automatically be assigned as Mac OS Extended (Journaled). If you made more than one, you can resize them by dragging the divider line between the partitions, or clicking on a partition graphic and manually typing in a size at the right. When you have everything the way you want it, click Apply. This will only take about 20 seconds.
You can then quit Disk Utility, which will return you to the DVD's desktop and continue on installing Snow Leopard.