TFTP - problems

Dear everyone,

I am a novice and don't know how to deal with this. A few minutes ago
my DOS prompt started itself and someone was issuing commands remotely
on the command prompt. It was so quick that I don't recall much
details but it did show the message....
'Transferring data using Tftp services..."

After reading about this on Google, it is evident that someone is
using my system to transfer files to and fro from my system.

Can anyone please throw some light what can I do prevent this from
happening? Any help whatsoever...any articles on web.....

PLEASE HELP...!! GETTING WORRIED....

Thanks..