No.Does re-installing windows definetely get rid of a virus?
Some types of malware that hook in to the boot sector (eg. such as some hypervisor style nasties) can persist through conventional disc formatting, as that does not erase the boot sector. Reinstalling the partition table and boot code should, though.
Reinstalling windows - fully, booting from external media and formatting or recreating all partitions in the process - will eliminate slowdowns cause by file or registry corruption, though.
As you have been told, you have not got a "virus" from a USB stick unless you have disabled all security in the computer. Malware from an online source, or simple windows system corruption, is near infinitely more likely.