|"And possibly you can answer why this laptop's c drive is so much smaller than the d drive?"|
My guess is someone has probably either re-installed just Windows, or re-installed the original brand name software installation software. E.g. You are not allowed to install Windows on a partition if Windows has already been installed on it, unless you delete the partition data contents first, so people often delete the contents of the smaller partition on a brand name hard drive and install Windows on the smaller one. Often when they find the new Windows installation works fine, they then delete the contents of the larger partition and re-format it, or they just re-format it.
Vista and Windows 7 always see the first installation of it's Windows partition (the one that has the full \Windows folder on it) as being on C, so when the original Windows partition was deleted, then Vista or Windows7 probably sees the reamaining one Windows installation as being on C.
In Vista and Windows 7, you can easily change the size of the partitions on any one physical hard drive, if they were made by Vista or Windows 7, in the operating system itself, in Disk Management.
In your case...
I'm assuming there are only 2 partitions on the one physical hard drive.
(If there are more than two, you can only change the size of partitions that are beside each other)
You must have lots of free space on the larger partition.
You shrink that partition's size on the end of it next to the smaller partition such that the freed up space becomes un-allocated, leaving at least a reasonable amount of free space on that partition that was previously larger.
Then you increase the size of what was the smaller partition such that all the un-allocated space next to the prevously larger partition is used for it.
It's recommended you back up the personal data you don't want to lose before you do that (copy it to a location other than the hard drive you're fiddling with) , but that procedure usually works just fine.
It's recommended that you do not have any extra programs running at the time, and that you DISABLE any resident modules - a part that runs all the time looking for suspicious activity - of any anti-malware software you have installed on the system, BEFORE you do anything major like that.
If you don't know how to do that, tell us which anti-malware (anti-virus, anti-spyware, anti-trojan, etc.) software you have installed.