Well, I was in the same situation when I first bought my Toshiba laptop...
First thing I did was wipe the whole disk (including the Recovery partition) and reinstall Win7 from a typical OEM disk using my laptop's serial number. I downloaded all needed drivers and software from Toshiba's website and installed just what I needed to get the laptop working with Win7. Nothing more, nothing less. I'm not saying that you should do the same but think about it...
1. If you ever have to reinstall Win7 (and you will, believe me) you will have to use the Recovery option. This option, in most cases, will wipe the whole disk and bring it back to the state it was when you first bought your laptop. Not very practical, right? In your laptop's manual you will find more about the recovery procedure. I suggest you read it.
2. Most preinstalled Windows come bundled with a bunch of programs provided by the laptop's manufacturer. Some of them are useful, some of them are just wasting resources. Are you sure you need ALL this stuff? Why not choose what you need and download it from the manufacturer's website?
Having said that, and if you decide to keep Win7 as it is, the first thing you must do is make room in your disk by shrinking the Win7 partition. There are many linux partitioning programs you can use and PCLinuxOS has one of the best (diskdrake) but I suggest that you do it from within Win7 since you are dealing with NTFS.
Go to Control Panel -> System & Security -> Administrative Tools -> Computer Management -> Disk Management
With this tool you can manage your partitions. Right click on the Win7 and select "Shrink volume"
This will allow you to reduce its size to your desired amount (which depends on how many programs you are planning to install).
You should create one big partition (i.e. "Data") where you will store all your files. This can be used as PCLinuxOS' /home partition too.
PCLinuxOS needs at least 3 partitions: one called root (/) where the OS and all the apps will be installed, one for swap (/swap - virtual memory in Win7) which I like to have it equal to my system's RAM, and one more for your configuration and personal files called /home. As I mentioned before, this partition can be shared with Win7 but if you prefer you can separate it. During installation, PCLinuxOS will create folders inside /home such as Documents, Music, Videos etc.
20GB for / (root) should be more than enough. I have 15GB, a lots of programs installed and only half of it is used.
Swap should be equal to your system's RAM. Mine is set at 4GB. This particular partition can also be used by other Linux installations if you plan to do any in the future...
You should use ext4 as the filesystem of your root partition and NTFS for your /home if you decide to share it with Win7 (otherwise it should be ext4)
That's it for now... Hope I gave a good head start... Come back if you have more questions, I'll be glad to help...