Place the Win7 Installation DVD which says bootable in the DVD (not CD) drive, restart the machine and boot the DVD:
How to Boot your Computer from a Bootable CD or DVD
If you don't have a DVD drive, borrow or buy one or use another computer's DVD drive to extract the ISO from DVD using ImgBurn, then write the ISO to flash stick using Ultra ISO trial version: Open ISO on File tab, Write Disk Image on Bootable tab, Format, Write. Boot flash stick under USB or HD's.
On the second screen select Repair My Computer, accept any offered Repair, if it doesn't start boot back in to run Startup Repair from the DVD Repair console's Recovery Tools list. System Restore should be included in Startup Repair but you can also run it from the Tools list.
http://www.sevenforums.com/tutorials/105541-startup-repair-run-3-separate-times.html
If It won't start after three separate startup Repair attempts with reboots, confirm that the Win7 partition is still marked Active and run the Repairs again:
http://www.sevenforums.com/tutorials/71432-partition-mark-active.html
If it still fails you can copy out your files using DVD with this method:
http://www.sevenforums.com/tutorials/93347-copy-paste-windows-recovery-console.html
Then clean reinstall after wiping the HD first to overwrite any possible infected or corrupt code, following these tips to get a purrrfect install:
http://www.sevenforums.com/installation-setup/125874-re-install-windows-7-a.html#post1086729