I want to install Ubuntu alongside with Windows, partitioning the system. I readed some tutorials online and it was said that I can have only 4 primary partitions. My windows 7 system shows me 3 partitions already: OEM partition, Recovery partition and OS(C:) partition. Is it still possible to install ubuntu satisfactorily?
Yes it is. Although you can only have 4 primary partitions you can have as many logical partitions as you want. Logical partitions are basically the same as primary you can install Ubuntu on those.
Just create logical partitions for your Ubuntu.
When Microsoft developed it's 'new' partition table type around the 1980s it restricted the number of partitions a hard-drive could have to only 4. These have become known as "Primary Partitions". It would be inaccurate to think of this as the main episodes on a Dvd because these could be shorter than the bonus features.
Some clever people managed to make one of these a new type of partition that could hold a large number of other sub-partitions. This new type of partition became known as an "Extended Partition" (it should have been "Extending" not "Extended" but nevermind). Perhaps think of this as the bonus features menu on a Dvd. When you select "bonus features" it doesn't play anything, it just gives you another menu.
The sub-partitions became known as "Logical Partitions". When people quote the number of partitions on a drive they usually exclude the Extended Partition. Perhaps think of these as the the bonus features on a Dvd. However, the bonus features could be a lot longer than the movie or the episodes.