What you want to do may be easy or may be just about impossible, depending on how you have used the 2nd partition on your first hard drive.
First, here?s a procedure for doing what you want; it?s not difficult. The question, which I will address separately below, is what the implications of doing this will be. CAUTION: Be sure that you understand this procedure and what each step does and why. Your system may differ from what I assumed based on the information you supplied (which is, in at least some respects, incomplete), and if you just follow the steps blindly, without understanding what they do and why, you MIGHT destroy things that you want to keep. I strongly suggest that before you do any of this, that you back up EVERYTHING in both partitions on your 80GB drive.
For the sake of argument, I will assume the following drive assignments:
Drive C: is a primary partition on your 1st (80GB) drive.
Drive D: is either another primary partition on your 1st drive or a logical drive within an extended partition on the 1st drive (it almost doesn?t matter).
Drive E: is your second (250GB) drive.
Before starting, verify that you have enough free space on Drive E: to hold the contents of Drive D: (the partition that you are going to eliminate)
Step 1: Create a folder on Drive E: called Drive_D (e.g. E:\Drive_D\).
Step 2: Copy all of the contents of Drive D into the folder you just created on Drive E: Verify that you have EVERYTHING; the number and sizes of files and folders in the folder on Drive E and Drive D should match.
Step 3: Using either a 3rd party partition utility (see below) or Disk Management (part of the Computer Management utility that comes with Windows), delete partition (or logical drive) D and, if present, the extended partition containing it.
Step 4: Using a 3rd party partition utility, expand Drive C: to include the space freed up when Drive D: was eliminated. The two recommended utilities for doing this would by Partition Magic (Symantec/Norton), or Disk Director Suite (Acronis). There are some other products that may be able to do this, but these are the two that I would recommend. It is not possible to do this with only components of Windows, you will need a 3rd party product of some sort.
Step 5: Copy the entire contents of the folder you created on Drive E: (e.g. the folder E:\Drive_D\) back to (the now larger) drive C:
Some versions of some 3rd party products may be able to combine all of these steps in a single operation of combining partitions. Either way, actually doing this isn?t difficult.
Of more concern, and really more complexity, is the implications of doing this.
In doing this procedure, you will have moved the files that were originally on Drive D: to Drive C:.
In general, installed PROGRAMS can?t be moved and probably won?t work (either at all or correctly) after the move if you do move them. However, most people don?t have programs installed on drives other than C: (specifically, in C:\Program Files). If you do have programs installed on D:, the best procedure is probably to remove them before doing any of this, and then reinstall them after the move (you will need your original program disks). However, you may lose settings and customizations that you have made. In some cases, programs installed on D: may have valuable, even irreplaceable data in their program folders on D: also. For this reason, if you are going to do this, make a complete backup of D: before doing ANYTHING.
While data is less problematic than programs, there is a chance that moving data files from D: to C: will cause problems with some programs that think that they know where some data files are located. There is very little guidance that I can give you on this subject because it is so highly program specific. This isn?t likely to be a major problem but it does have the potential to be with some (fortunately few) programs.
I hope that this helps you out.