How can I allot free space on my hard disk to an already existing drive in Windows?

You will need a partition-management utility for this purpose. Allocating free/unutilized or unallocated space to a partition or reallocating space from one partition to another is a fairly easy process. However, as a precaution, it is recommended to backup all important data before-hand. (I never do that since I have uninterrupted power supply and a good quality hard disk that has never failed, but that should never stop anybody from backing-up their data prior to repartitioning their hard drive).

You will have to do these:-
1. Download a decent partition management utility. I recommend EaseUS Partition Master Home Edition ( http://www.partition-tool.com ). It is free, powerful and very simple to use.

2. Install EaseUS and run it. You will see all your hard drive partitions and unallocated space (if any) in both detailed and graphical views. To make more free space in your C: drive, you need to have some unallocated space adjacent to the C: drive. Look at the drive structure in EaseUS, and see if you have any unallocated space before C: drive. If there isn't any unallocated space just after or before C: drive, you can always make some (read point 5 before reading points 3 & 4).

Note: If you find a small System Reserved partition, you should to leave that untouched, since it is used by your operating system and altering it may cause boot problems.

3.(a) If you find an unpartitioned space next to the C: drive, click on the C: drive (from within EaseUS, of course) and click on the Resize/Move button. A "Resize/Move Partition" dialog will open, asking you to decide size and position of the C: drive that you would like it to have. You may move the slider-ends to include all of the adjacent free space(s) or you may type in the size of the new C: drive in the relevant boxes.

3.(b) Alternatively, you may first create a new partition from the unallocated space by clicking on the unallocated space and then clicking the "Create" button. Then, you can merge this new partition with the C: drive by first clicking the C: drive (not the new partition) and then clicking the "Merge" button. Note: You can only merge adjacent partitions.

4. After you have finished re-sizing your C: drive, click OK. You will notice the partition tables have readjusted to your preference. If everything appears as you want it to, click Apply to make the changes permanent. Your computer will restart and your partition changes will be reapplied. This process takes time depending on how much space is being (re)allocated. On successful completion, you will boot into windows. Open My Computer or Disk Management to see your current partition structure. And you're done!!

5. If there isn't any unallocated free space around the C: drive (which is the most probable case), you will need to shrink the drive just next to C: (usually lettered 'D:') in order to add that new space in C: drive. This is what I do when I need to make more room in one particular drive. To do this, click on D: drive (or whichever drive is next to C:. I'll assume it is lettered D:) from within EaseUS Partition Master and click on the Resize/Move button. A "Resize/Move Partition" dialog will open, asking you to decide the size and position of the D: drive. To make space that will be added to C: drive, you will need to move the slider from the LEFT end in order to make unallocated space BEFORE D: drive (since C: drive lies on the left of the D: drive ;) ). Determine how much space you need to add in C:, and adjust the size of D: accordingly. You may also grant all of D: drive's free space to C: if you want (I wouldn't like that on my computer). When done, click OK. You will see an unallocated space has appeared next to C: drive. Now do the steps in point 3 and 4 above to increase the volume of C: drive!

There is a popular open-source utility called GParted that comes in its own Live CD and also with many popular Linux distributions like Puppy Linux. You may also use that to reallocate space on your hard disk. But seeing that you are already using Windows, i feel EaseUS will prove to be the easiest method.

Hope it solves your problem. :)