how to format my memory card?

Apparently, you performed the correct steps. If Windows report any error, it can indicate that the card have some defect or some other problem.

For a 32Mb card, 30.6Mb free is about correct. Some of the capacity is used for the root folder and alocation table.

You don't need to format the card often. Usualy, you only do that one time.

Now, for the eventual problems:
It can be dirty or coroded contacts. First: insert and remove the card several times. If that's not enough:Use a pen eraser and gently rub the contacts.

The adaptor may be defective. In this case, any card will beheave eraticaly or it can cause systen slowdowns and instability. It will cause problems with any inserted card, not just one, and possibly also when there are no card inserted. Replace the adaptor.

The card itself may be defective. If the remaining capacity is enough, you can use it for non-importent data. It's beter to replace the card by a new one.

Electro
October 2013