I have just build a new computer ( my first one ), and i have forgotten to partion the hard drive off, and i have installed the OS onto the harddrive, so i was wondering do i actually need to partition the hardrive and also, can it still be done with the OS on, or do i have to completly re0install the OS, like i said this is the first time i have built a computer, and i do not fully understand the partioining bit, so please go easy on me thanks
If you installed an operating system to that drive, you also partitioned it. This happens during installing, whenever windows asks you where to install it and you can create partitions or simply select the "unused space" and windows will create one big NTFS partition for you.
So no, everything is fine you don't need to reinstall.
You are probably OK as sub mesa says. The only unanswered question is whether the Partition it DID create for the installation used all the space available, or whether instead it left some Unallocated Space that needs to be Partitioned also so it can be used.
Look at the C: drive you have in My Computer. RIGHT-click on it and select "Properties" to see what its total size is. It will probably look like about 93% of what your Disk Size was specified. For example, a 500 GB HDD will show up in My Computer with a total space of about 466 GB, and a bit of that of that will already be used up for files. A 1TB drive may look like it has 910 GB total space. This is ONLY because Windows uses a different way of calculating a "Gigabyte" - it is NOT missing any space!
So, if your C: drive has about the right total space, compared to what your HDD specification was, then ALL of that drive already is in the first Partition and you do not need to do anything else. If not, there is a simple procedure to use Unallocated Space to create another Partition to be used as a separate drive. If you think that is your situation, post back here and we can advise how.