If you know the model, try to go to the Asus site and look for downloads for your specific model. Sometimes they'll have a download (or CD mailed to you). Otherwise, you could try a Win 7 disk, not sure if they provide ISO downloads on the Microsoft site or not,but you can torrent them pretty easily.
I would have a feeling that your MBR was destroyed. Try:
Put the Windows disc in the drive
Restart your PC
Enter your BIOS
Change the boot order to boot from CD/DVD first
Save and restart again
Choose your language, then click Next
Click Repair Your Computer
Choose your operating system (there will likely be only one listed), then click Next again
(Alternate step, skip the first time) type BootRec.exe /fixmbr and press EnteR
Select Use Recovery Tools That Can Help Fix Problems Starting Windows and click Next
Click Startup Repair
If yo're going to try the fixmbr, you can skip the last two steps too.
You can make a Windows recovery disk from a good PC running Windows 7 as well:
How do I Make a Windows Recovery Disc?
You can use that if it's a hassle getting a real Win 7 disk.