Follow the above steps posted by the others to remove the infection then backup your data.
I am a Dell Community Rockstar and have been focusing mainly on Installation and Setup of Windows on the Dell Community Forums. Please see my Windows Reinstallation Guide:
http://en.community.dell.com/support-forums/software-os/f/3524/t/19480753.aspx
If you still cannot boot then boot from a Linux Live CD and recover your data. To do this see Data Recovery of the HardDrive Using Fedora (page 412).
Have a look at A Clean Install of Windows 7 (page 6)
If you can backup your data from within Windows follow Step 1c and 2 and 3a.
Have a look at Step 1b of A Clean Install of Windows 7 and download the Windows 7 Home Premium 64 bit with Service Pack 1 .iso and make either a DVD or a USB (USB is generally recommended due to lower burning errors). Check the 25 Digit Product Key on the
COA is readable and write it down.
You won't want to perform Step 3b of A Clean Install of Windows 7 as you have a bad installation.
Carry out Step 4 and 5d using another computer. Also have a quick read through Step 5a and 5b. Prepare SATA Drivers in Step 6 if necessary (state your model of Dell for more details).
Step 7a you are best to prepare the DBAN disc using another computer. I recommend using it on your infected computer to securely wipe the hard drive and remove all traces of infection. Step 7b likely won't apply to your model but I would need to know what
model of Dell you have for more details.
Follow through with Step 8. 9 and 10.
Step 11 either activate using the Activation Backup Recovery Program Step 11b or Step 11e using your 25 Digit Product Key and Phone Activation.
Carry out the checks in Step 12.
Step 13 is recommended.
Step 14a likely won't apply to your system. Step 14d you will likely have Dell Dock so have a look at Step 14e and My Dell Downloads.
Step 15 won't apply as you should have securely wiped your hard drive so there will be no Windows.old folder.
Step 16 is recommended.
Step 17 is optional but recommended. Step 18 and 19 are optional.
Step 20 is recommended for the future.
This should bring your system to full performance.