That would be like starting all over... you'll want to at least get any important files backed up, bookmarks, address book (if it's not browser based), etc. You'll also want to make sure those files are not infected before putting them back on your cleaned drive.
Do a google search and read up on it. It's good to be prepared before you start.
If you don't have the orig install disk(s), it's possible to create one from the installation files on the HD that very likely came with your computer. Plenty of people do it, but there's a learning curve involved. Also, there's always the nagging concern that a trojan is lurking in those files; probably unlikely since the files are just sitting there, not running any apps or the system, but still ...
I've heard that you can call MS Tech Support, prove that your system is legit and explain to them what you're trying to do. They will send you the installation CD's (MS Office too?). You might also want to lobby them politely to allow the customers (us) of their OEM customers (Dell, etc.) to request that we be able to have physical OS CD's/DVD's at the time of purchase at no extra charge.
I haven't even mentioned backing up all your data since that's so basic.
The actual process of reinstalling the OS isn't difficult, but you need an overview of the process. There are plenty of helpful sites that can walk you through it.
Once everything is backed up - data only w/o viruses - you might want to look at Partition Wizard. You can burn a boot disk and wipe your hard drive (delete all partitions at the same time). Then use the Win7 Install Disk to partition and format the drive and, of course, install the O/S.
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