I did this a while ago when I swapped out my motherboard - just chuck in the DVD and select Repair (NO NOT select new install!!)
Only thing to be aware of is several of us have found that if your OS drive isn't the 1st drive the system finds, it may not offer you the option to repair. Make sure that your OS drive is the Master drive on the 1st SATA port. Also, you should have all of the drives attached when you repair or you may lose the data... can't confirm that but it's what I was warned about.
Once that's done it's just a matter of re-installing your Add-ons and you're away - was simple once the drive issue was solved!
Only thing to be aware of is several of us have found that if your OS drive isn't the 1st drive the system finds, it may not offer you the option to repair. Make sure that your OS drive is the Master drive on the 1st SATA port. Also, you should have all of the drives attached when you repair or you may lose the data... can't confirm that but it's what I was warned about.
Once that's done it's just a matter of re-installing your Add-ons and you're away - was simple once the drive issue was solved!