Originally Posted by eyesilver
i have 640GB hard drive acer aspire 5750 intel core I5 which i have 3 partitions on it ...
... during the installation type the device named into unusable although it diplayed free space of 150GB & from that space i wanted to add a logical & primary partion manually ...
I think this is your problem.
You already have 3 Windows partitions, which you have identified as C, D, and E. I would assume that these are all primary
You are trying to create one additional logical partition plus another primary
partition. You can only have 4 primary partitions. If you have 4 primary partitions you can not add an additional logical partition.
If you have three primary plus one logical partition, you can not add an additional primary partition.
Choose "something else" during the partitioning part of the Ubuntu install.
In the unallocated space that you have created, create two logical
partitions. One will be for the root partition and one will be for the swap partition.
If you have a good amount of memory, you can set the swap partition to 1 GB. The rest can be the root partition.
You could also go with three logical
partitions. One would be for root (about 20GB).
One would be for swap (1 GB).
The rest would be for the home partition, which is where all of your data and user settings are stored.