There's a wonderful company called Google, who run a web search tool at www.google.co.uk or www.google.com.
I went to it and typed in "USB sound card" and got a whole stack of results.
Try this: http://www.google.co.uk/search?source=ig&hl=en&rlz=&=&q=USB+sound+card&aq=f&aqi=&aql=&oq=&gs_rfai=
Actually I'm surprised your computer has no sound card. Most do these days.
Do you know exactly what Make & Model PC you have?
Do you have a manual - have you read it??
There's a really handy utility called System Information for Windows (it's free) which you can get here: http://www.gtopala.com/
You can run that and find out book, chapter and verse about what hardware you have.
You never know, you may even discover you have a sound card.
If so, you may just need a driver. If you do, try the maker of your PC or use the info from System Info for Windows
Google is your friend.
Why do I bother?
May 2010