You should have the HD set first to boot, then when you need to boot the DVD use the one-time BIOS Boot Menu key shown on first screen.
I don't believe Samsung provides a Windows 7 reinstallation DVD, but instead you may have Factory Recovery DVD. Can you say for sure? What does it say on the disk exactly? Did you buy Windows 7 from Samsung?
If the licensed OS which came with the PC and is printed on the COA sticker on bottom with a readable Product Key is Home basic, then download Home Premium from Step 1 of Clean Reinstall - Factory OEM Windows 7
, run the eicfg removal tool
to unlock all versions, burn an all-versions installer to DVD or write to flash stick using the tool provided. Then choose your licensed version during Clean Install, do a Custom Install deleting all partitions using Drive Options in Step 7 of Clean Install Windows 7