controlling Powered USB?

[mb26]

my new phone uses the USB data cable to charge up the phone.. this seems like a great idea, but ur meant to fully charge>fully discharge etc to get best battery life..
+ when its plugged into the computer i can't access the files on (from) the phone, so have to disconnect it.. this leads to short charge>discharge cycles which aren't good..

So, i would like a way to get the PC to separate the power from the data?;
if i could set the PC (XP) so the USB ports (or a particular one) doesn't supply power for when i want to transfer data that would be great..

and/or;
if i could get the PC to 'forget' the device was there (i tried safely remove HW- it is seen as a removeable drive) then it could continue to charge but allow me to access the files from the phone

[mb26]

anyone?