By USB I assume you mean a USB Flash Drive? These devices are great for transferring files from one computer to another, but are not good to be the only backup device.I have started transferring individual files to a USB
As Masterchief said, You should get a USB external HDD and Drag and Drop/Copy and Paste all of your user files under C:\Documents and Settings to it. Do not backup programs.
The other option is to clone your whole computer C: drive to another HDD. Then swap the drives around. If the computer boots with the cloned drive, then you are good to go. You can use the free Acronis for WD drives or DiskWizard for a Seagate drive.