You don't say what operating system you are using, but yes you should be able to instal a USB card as long as you are careful.its not a difficult job.
If you have Win XP then it's no real problem. Win 98 or ME, in my experience, have always been a bit more problematic but not difficult. Key thing is to make sure that you load the new drivers first.These you will get usually on a CD but may have to download.
Ensure everything is unplugged before opening up your PC. There are usually screws on the back which release the case, sometimes thimb screws, others a screwdriver is needed. Touch a radiator or similar electrically grounded object first before touching anything inside the box. Remove a blanking plate on a spare slot for the new card and insert it carefully, making sure it is pressed into the connectors firmly. Put things back together, and reconnect and repower PC. New hardware should be found and new drivers installed. Job done.
If you have existing USB1 ports I found that because of the new drivers there is often a clash and these become inoperable. Perhaps someone else has found a solution if this occurs. Good luck.