That's a good sign. The only reason I asked is that it is possible for computers on a firwalled network to pick up strange things from other machines on the same network. Frequently LANs are less restrictive with machines within than they are with machine's outside (so if someone on one of your networks has been lax in updates and virus scanning you can potentially pick something up simply by going onto the LAN they use).

If you have trouble booting from the USB drive, one potentially simple way to test your system is to boot up from a Linux Live CD (or a bootable Windows CD). I've had to salvage data from many laptops that had dying hard drives and using one of those CDs is a great way that I've found for checking the system in general. The boot-up process tends to be very descriptive and if there is a problem with any of your other hardware the Live CD would pick it up and even if it failed and couldn't boot you can often see with exactly which component it had problems.