yDecode works with ANY newsreader - it only supports automatic configuration for most popular newsreaders such as Outlook Express and Windows Mail but manually you can configure it for any newsreader. To verify if you need yDecode - check if you can actually read yEnc posts - do you see scrambled content or decoded picture (the best is to test with pictures)? If you see scrambled content your newsreader most probably doesn't support yEnc - therefore you need a decoder.
Note that yDecode is not only yEnc decoder. It is also multipart auto-joiner, it also saves attachments into a folder of your choice etc. so only having yEnc in a newsreader still may not mean that you don't want to use yDecode as it can do additionally simplify newsgroup browsing through its features.
