MP3 files are usually encoded at a sample rate of 44.1kHz. Occasionally, however, you will come across something different (48kHz is the second most common). Though they run on most computer programs, non-44.1kHz MP3s may not work on certain portable devices. Luckily, converting from an irregular MP3 to a standard 44.1kHz version is simple and can be done using several programs.
Convert the MP3 to 44.1kHz with iTunes. The conversion process is easy, but first you need to set it up properly. Hit CTRL-[comma] to open Preferences, then click "Import Settings." "MP3 Encoder" is what you should select from the first drop-down box and any setting but "Custom" will work for the second. All the predefined settings encode at 44.1kHz.
Convert the MP3 to 44.1kHz with Winamp. You know you're safe with this program because it will not convert to anything except 44.1kHz. Right-click the file and select "Send to -> Format Converter." Then just be sure you're sending it to MP3 and the sample rate will take care of itself.
Convert MP3 to 44.1kHz with Windows Media Player. The advantage of this option is that you already have the program on your computer. The disadvantage is that first you have to burn the MP3 files to a blank CD, then rip the CD. If you have CDs to spare, though, this way is quick and simple. Be sure "MP3" is selected in the box that pops up when you click "Rip" though. Otherwise you will convert your song files to WMA.