Try this: http://www.blisstonia.com/software/WebDecrypto/ & this: http://sharkysoft.com/misc/vigenere/
If this doesn't work, notice where it says 'Key' and 'Clue'? This is where the cache owner might have made it a bit more tricky. Scout the cache page and see if the cache owner has said something like: 'The key to solving this cache is perseverance' In that case, type perseverance in the Key/Clue box and see if that helps!
