You have 2 HD's of each 20 GB?
First thing: don't use larger chucks of HD than 10 GB or so. So you could best partition them. Make 4 partitions of 10 GB or something like that. Or keep one at 20 GB. Keep things orderly.
But you're saying that Windows doesn't see those HD's as 20 GB, but as 7.87 GB.
Since when did this happen? Did you install them new?
What I think is the problem is that the HD's aren't completely formatted: 7.87 GB is probably formatted, but the rest isn't. So you can use Partition Magic or some other app to format the rest. If the rest isn't formatted, Windows won't recognize it...so it has to be formatted in FAT, FAT32 or NTFS.