I just downloaded Ubuntu 11.10. When trying to install it, surprisingly I saw that it didn't detect my Windows 7 HP SP1.
I have this windows pre-installed on my net-book.
can anyone help me with this problem?
It is indeed fairly common that a laptop or netbook that has Win7 on it already has 4 primary partitions on the drive.
I can't say for sure that this is your problem, but let's assume it is.
If you can see there is an option in the installer missing, then you can presumably boot into the live environment. That is the "try without changing" option.
Do that and start Gparted. That is a partitioning program that can tell you what is on your drive. Have a look and see how many partitions there are. On a Win7 install, I gather it is fairly common to have 2 partitions that are directly related to the Win7 install A recovery partition A partition containing tools from the manufacturer of the laptop.
I don't remember exactly, but I think I blitzed the recovery partition on this netbook to make room for an extended partition to enable me to make logical partitions for the Ubuntu install. That is the principle, at any rate. Have a look what is there, maybe use the "create a recovery medium" facility (I didn't), and decide which of the existing partitions you can afford to get rid of.
If there really are already 4 primary partitions on the drive, then you do have to get rid of one of them in order to install anything else on there. Either that or get rid of the Windows install completely.