It seems likely that the HDD has one Partition of 30 GB established and Formatted on it, so that is what you can "see". There MAY be another Partition you have not detected yet, OR the remaining space may never have been allocated. The Windows tool to examine this is Disk Management.
Click on Start at lower left and RIGHT-click on My computer, then select from the mini-menu Manage. This opens a new window. In its left column expand Storage if necessary and click on Disk Management. The right side will show two panes, and each of them scrolls so you can see all they hold. The upper right pane show you all the drives Windows can use right now. The LOWER RIGHT pane is where you can see those plus any other devices that Windows does not yet understand.
In the LOWER RIGHT pane each drive is represented by a horizontal block; hard drives have a small label box at the left end with things like "Disk_0", a disk type, a size and a status. To the right are one or more sub-blocks, each representing one Partition on the HDD unit. Each Partition has its own letter name and is treated by Windows as a separate "drive". The Partition will have a Volume Name like "Boot Disk" or "Harry's Drive", a letter name like "C:", a size of the Partition (not the whole HDD), a File System type like NTFS, and a status. So, find the main block representing your 100 GB Maxtor. It should show you one Partition block for the 30 GB "drive" you can use now. But what does it show to the right of that? Is it just one big empty block marked "Unallocated Space"? If so, you can use that space in either of three ways (below). If, on the other hand, there is another Partition there but it has no letter name, you can fix that by RIGHT-clicking on the Partition and choosing to Change its Name to some letter not already used. If you do this, back out of Disk Management and reboot so Windows can update its Registry files. Third possibility: if the extra Partition has a letter name, but its File System shows as "RAW", then there is a little corrupted data in its Partition Table and Windows cannot figure it out. What you do depends on what you know about it. If it is RAW but you are sure it contains no useful data, you can clear off that one extra Partition and re-create another. However, if you know it has data you want, you need some data recovery software.
BUT if all that extra space is just a block of Unallocated Space, you can start to use it in either of three ways. IF your Windows allows it (some do, some don't) you can RIGHT-click on the FIRST Partition that you are already using (30 GB) and choose to Extend it into the adjacent Unallocated Space, using up all of what's available. Then back out of Disk Management and reboot to see your new larger drive. The second straightforward option is to RIGHT-click on the Unallocated Space and choose to Create a new Partition from it. Specify the size you want, and it does NOT need to be bootable. You will have to Format it also - either now, or as a separate task after Creating the Partition, depending on which Windows you have. For the format operation, these are the typical options to set: use the NTFS File system and have it do a Full Format (not quick). The Full Format takes a VERY long time (several hours) to do its job, which includes a complete test of the drive area for any bad sectors. When it is done, you back out of Disk Management and reboot, and the new "drive with its own name will show up in My computer.
The third option, IF you don't want the data in the 30 GB Partition OR if you can back it up somewhere else first, is to Delete the 30 GB Partition so that the HDD unit is all Unallocated Space, back out, and reboot. Go back into disk Management and this time Create a new Primary Partition that uses ALL of the drive, and set options as above. I do NOT recommend this if you are using this drive now as you boot device, unless you know how to re-install your Windows OS and software, or how to Clone it.