It sounds like you have the Install DVD. But if you have done the SP1 update, does it include that? If you already have a retail install DVD, you can download a version that includes SP1 from here.
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Download it and burn the .iso file "as an image" to make a bootable DVD. You should then be able to do a repair install and keep your programs and data. Just make sure and do the Update, and not Custom Install.
There is never a 100% guarantee something will work. Backups are always a good idea.