If your PC is currently running Windows only, the chances are very, very good that Windows will occupy 100% of your hard drive. In order to install Linux as a dual boot, you will need free, unallocated space on the hard drive. Note that *unallocated* space is not the same thing as *unused* space. To illustrate the difference, if we assume you have an 80G drive, it probably is partitioned as one single partition, which has all been allocated to Windows. Perhaps you've got 25G of data, with 55G of space remaining (meaning it's unused). What you'll want to do is resize that existing 80G partition, to make it smaller and thus create some free (unallocated) space for Linux -- just like Windows has its own partition, Linux likewise will need its own partitions.
To illustrate, you might decide to leave Windows in a 50G partition, and give the remaining 30G to Linux. You would then resize your partitions, and the next time you fire up Windows, you would see that your C:\ drive's size is now 50G instead of 80G, while the other 30G is free (as in unallocated) One thing you want to avoid is shrinking the Windows partition so small that it cripples your ability to use it - for instance, you wouldn't want to shrink the Windows partition down to say 26G, because that would leave you with only 1G of available space (since you've already got 25G of existing data)
One good tool for resizing partitons is BootIT NG
but there are others out there. As hpladd mentioned, it's a good idea to defrag your drive before starting, as that will maximize the possible space available to Linux. After the resize, start the Linux installation, and specify that you want to install it into the unallocated space. My advice would be to decide in advance how you want to divide that hypothetical 30G, but you might go with a 512Mg swap, a 10G / and the rest for /home. Partitioning questions are one of the most common questions here at LQ, and everyone's got their own opinion as to what's best. There's no real right or wrong. Good luck with it either way