First of all, sorry for taking so long to respond. I don't normally come to this site anymore.And it is not "Mr" Doreen It is MISS.
As for booting your laptop using "F8" you need to press that when you first turn on your Laptop when you see the "setup" option on the screen. this usually appears right after the Dell logo.
Anyway. try pressing "F10 or F12" when you see the setup option. You'll need to hold it down for a few seconds or keep pressing it quickly till the "boot option" screen appears. This will give you the options to either boot from your hard drive, USB, CD etc... If you think there is a problem with you laptop, put in your Windows 7 recovery disks (if you've made any) or just try using the windows 7 install disk. using the "CD" option or if you've got them on a USB drive then choose that option.
Viruses can cause your PCs to act up and not properly boot as well. My advice would be to scan your laptop once you've been able to properly boot into your desktop using any anti virus programs you have. Also clean out you temp files, recycle bins etc.. then defrag it. Any PCs are like cars, you need to maintain them regularly for them to work properly.