Windows 7 USB Flash Drive Backup

I would like to get a disk image from my clean computer onto a Flash Drive for a backup in case it might be needed for a reinstall in the future. What is the easiest way to get it done, for the computer simple minded (me). Thanks. *S

Dell Optiplex 755 Mini Tower Desktop PC
. Intel Core 2 Duo Processor 2.3GHz
. 2GB DDR2 RAM
. 160GB / 5400 RPM SATA hard drive
. Intel Graphics Media Accelerator 3100
. DVD-CDRW
. Windows 7 Home
.Dimension:17.5"L x 7.3"W x 16.1"H, 25
lbs.