Winamp will not play certan file types HELP PLZ!

[Celena]

First of all let me say thank you for reading and let me apologize i do not understand much of the tecnical stuffs, i know how to use a computer not how they work so bare with me.
I have an HP vista and the latest version of winamp i only have the shoutcast DSP plugin and thats it for extras, not long agio i had a virus issue and my comp was reformatted and my files placed back on. Sence that time "MPEG Video Format" & "AVI Video Format" files no longer play in winamp but they used to and they work in windows meida player just fine. Is there a plugin or something i need to make them work? Or does winamp no longer support these file types? Everyone i ask just says "well convert them to this or that" ummm dunno how i would need step by step instructions. So can anyone help me? TYVM for you're time.