I would first try - Make a new User account - log into this and see if iDVD behaves.
If so the problem most probably is iDVD pref file (trash this and iDVD makes a new fresh on)
(log into the old account first so You trash the right one)
Not knowing the origin to Your problem - General approach when in trouble is as follows:
• Free space on internal (start-up) hard disk if it is less than 10Gb should rather have 25Gb
• Hard disk is untidy: Repair Permissions, Repair Hard disk (Apple Disc Util tool)
• Delete iDVD pref file - *or rather start a new user/account* - log into this and re-try
• Program miss-match: iDVD 5.0.2, Mac OS X.4.11 AND QuickTime 7.4.1 - is OK - DON’T work under Leopard
• Program miss-match: iDVD 6.0.4, Mac OS X.4.11 AND QuickTime 7.4.1 - is OK (might work under Leopard)
• Program miss-match: iDVD’08 v. 7.0.1, Mac OS X.4.11 AND QuickTime 7.4.1 - is OK (might work under Leopard)
• iDVD (08) v7 Locate theme folder. Move out iDVD1, iDVD 2 and eventually iDVD4 folders to desktop - re-try
iDVD 6.0.4 and iDVD 7.0.1 are compatible with Mac OS X 10.5 Leopard
iDVD pref file resides: Mac Hard Disk (start-up HD)/Users/"Your account"/Library/Preferences
and is named: com.apple.iDVD.plist
While iDVD is NOT RUNNING - move this file out to desk-top.
Now restart iDVD.