There are quite a few different programs that will help you sync your USB drive with the files on your computer. Probably the best-known tool is something from Microsoft, called SyncToy.
When you run it, it will help you set up two linked folders that you can then sync/update when you're using your USB on your PC. It's got a four-star rating on from CNET, which suggests it's a good piece of freeware that you can trust.
One drawback of SyncToy is that it doesn't automatically run, but this can be solved by creating a batch file to do that job instead.
Another option is to install 'Allway Sync', which has a dedicated version specifically designed for USB drives and portable hard drives, etc. Like SyncToy, it's a free tool, and whilst it doesn't have quite the same status as Microsoft as something you can trust, it does have a '100% Clean' rating on Softpedia, which suggests it's free from things like viruses and malware.