Actually it is possible to install w7 without the 100Mb reserved if you like. As you said it's for Bit Locker which isn't in w7 HP.
Here's how, whe you are installing, after all the loading files and updating stuff is done you are present with a window to select a language. At this point press Shift+F10, a cmd prompt will open.type in the following and press Enter after each line.
select disk n Where n is the disk you are installing to. There aren't names or drive letters so go by the size listed to select the right one.
clean or clean all (clean all writes 0's to all sectors and takes about an hour per 320 GB)
create partition primary
list partition This should show your partition you just created, if not..
select partition and select the new partition.
format fs=ntfs quick
Now you can complete the install and there will be one partition, C:\ for the OS.