Ringtones now MUST be in the .m4r format to play on the iPhone. I suggest doing this, which is what I did on my Mac, should be same on Windoze (Assuming you don't know anything for simplicity sake):
1. Make sure your Importing preferences are set on the AAC encoder: Preferences-->Advanced-->Importing-->Import using AAC Encoder, Setting on High Quality should be fine.
2. Drag your ringtone into iTunes, if it isn't already.
2b: If you haven't cropped your ringtone already: choose start and stop time if by getting info on a song, click on options tab, and choose start and stop time with the boxes checked)
3. Right click on the song, and select "Convert Selection to AAC."
4. Right click on the newly converted song, select "Show in Finder," then change the extension from .m4a to .m4r, accept changes. It is now a 'Ringtone' audio file.
5. Copy these .m4r files to your iPhone. You can do this via iToner, but I prefer ssh. Copy them to /Library/Ringtones/ and they'll be ready to go!
If you don't know what SSH is (Install OpenSSH via Installer.app, download a good ftp client (fugu is good on mac), find your iPhone IP, connect to it, password is alpine, then drag your files to appropriate location
Also, make sure there are no accents or weird symbols in the ringtones. I had a spanish song that had an accent over an 'a' and it wouldn't play. When i changed it to just a regular 'a,' it played fine. Go figure.