The simple answer is no it cannot be easily done. You can transfer files ok but the main programs are more differcult. I'll have a stab at explaining why.
When you install a program it goes to a certain file and all the parts of the program know where this file is and can find it. If you move the program certain parts of the program will look to where the program was installed initially and of course will not find it. You can move the Data you created easily enough its the programs that may cause the problem.
The choices are reload the programs from scratch on the 80 gig drive or check the second drive to see if it is slower and if so replace it with a faster drive and use a program like Acronis Drive Image to copy from the old drive to the new drive and then place the new drive in where the 160 gig one was. You could then perhaps buy a hard drive enclosure and put the old 160 gig drive in it and then you have a 160 gig external backup hard drive that could connect via USB 2.0.
To check the drives and their relative speeds look at the labels and look up the specs on the hard drive manufacturers websites.
See what anyone else suggests before you do anything.