Hi, there is no problem with the combo10.6.8 for 99.99999% of the people.
Do NOT attempt to use a Tiger/10.4 Disc to repair anything even if you had one.
"Try Disk Utility
1. Insert the Mac OS X 10.6 Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
*Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair Disk, (not Repair Permissions). Disk Utility checks and repairs the disk."
If it still reboots to the blue screen, reinstall 10.6.x
Then the big 10.6.8 Combo...
Repair Permissions afterwords, reboot.
I wonder if we could get away with the smaller 10.6.8 delta update that requires 10.6.7 now???
See if the Disk is issuing any S.M.A.R.T errors in Disk Utility...
Open Activity Monitor in Applications>Utilities, select All Processes & sort on CPU%, any indications there?
How much RAM & free disk space do you have also, click on the Memory & Disk Usage Tabs.
In the Memory tab of Activity Monitor, are there a lot of Pageouts?