1 Open a terminal (Applications -> Accessories -> Terminal)
2 In the terminal type "sudo gparted" without the quotes
3 In GParted, click on your Windows partition.
4 Choose "Resize"
5 Make it the size you want it to be
6 Hit OK or Apply
7 Click on the unallocated space
8 Click "New"
9 Make this partition's size be 1 or 2 times the amount of RAM you have. This is for your swap (pagefile)
10 Click OK
11 Click the rest of the unallocated space
12 Click "New"
13 Choose ext3 for the filesystem
14 Click Apply
15 Start the Installer
16 When you reach the part about partitioning, choose "manual partition"
17 Click on the partition that corresponds to your memory size (the swap one)
18 Click the edit button
19 Mark it to be used as swap
20 Click OK
21 Click the other partition
22 Click edit
23 Mark it to be used as ext3
24 Make it's mount point just a / (it's a choice in the drop-down menu)
25 Click OK
26 Click the checkbox on the line which is mounted as / if the checkbox isn't checked
27 Click the "Next" "Forward" or "OK" button to move on