Windows will also accept the hexadecimal key, no conversion is needed. In fact, it is the recommended format. From the documentation for configuring WEP in Windows XP:
If you are typing the WEP key using hexadecimal digits, you must type 10 hexadecimal digits for a 40-bit key and 26 hexadecimal digits for a 104-bit key. If you have the choice of the format of the WEP key, choose hexadecimal.
However, you should type
84127D134D083189A4AF970721, without the
: characters. Those are just separators which are not actually part of the key.
Finally, note that your key is now useless for security as you have published it on the internet.