I want to install ubuntu 11.04 from cd alongside windows 7.

I get to the choose partition screen, which looks like this.

Size Used
dev sda
dev sda1 ntfs 314 MB 105 MB
dev sda2 ntfs 301501 MB 39823 MB
dev sda3 ntfs 16106 MB 13282 MB
dev sda4 fat32 2142 MB 32 MB

When I click Install Now, I get this error message.

No root file system is defined
Please correct this from the partitioning menu.

In the Allocate Drive Space window, you will see the partition information. You need to select one to use or select unallocated space. If you select a partition by clicking it to highlight it, you can then click on the Edit tab to make changes. If you have unallocated space, you can click it to highlight it then click on the Add tab. In either case, you will get a new window and one of the options is Mount point with a box to the right and a down arrow which when you click gives you a number of options.

Unfortunately for you, you already have four primary partitions and all of them are windows partitions so installing Ubuntu on any of them won't work. You will need to resize one of your partitions (sda2 or sda3) which you can do from within windows. You will then need to delete one of the primary partitions and create an Extended partition in which you can create a logical partition on which to install Ubuntu.

Was windows 7 installed on the computer or did you install it? If you haven't done this before, there is a high likelihood you will have problems and lose data. I don't know why you have four partitions and don't know what would be on them. Boot, System and Recovery are fairly standard for windows 7 but I don't know what the fourth would be?

If this is a new installation, it might not be such a problem. Otherwise, I would definitely suggest you backup any data you want before starting.

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The above solution is good enough. But here is the alternate. You can install ubuntu inside windows by using wubi.exe. While running windows insert the ubuntu CD. Inside the CD you will find wubi.exe. Run it and you will find simple installer window. Choose the drive in which there is sufficient space. And give username and passwd of your choice and click on install. Then restart the PC then you will find ubuntu in the boot menu. Select ubuntu from the boot menu, it will take around 10-15 min for complete installation.

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As the partitions seemed important, a picture:

Picture of partitions

I've had wubi before, but wanted a real installation this time, on my brand new laptop, which I didn't install myself.

Can I go from wubi to full installation as rahulkya suggests? Just making sure I understood that correctly.

Not with your current setup. As I stated in my last post, you already have four primary partitions and they are all windows so your problem is as stated in my last post.

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I decided to go with wubi after all. But thanks for your help!

Don't worry installing from wubi is also full functional ..There is no difference between wubi installation and regular installation....
And there is a hidden advantage installing from wubi that you can keep a complete restore image of your UBUNTU..
When u have your installation complete, go to directory{in windows} where ubuntu is installed ..Search for root.disk..This is your whole / {root} partition.. keep this for future failures or restoration.... you can copy this whole file to any folder and replacing this with current root.disk will restore all your / partition at any time...

As Yancek said you may have a problem with installation as you're only allowed 4 primary partitions, only 2 of them have room to support another os & ubuntu's installer may or may not turn it to an lvm to keep all partitions active, before I would install with this setup I would surely backup my data.
Good luck.