Invalid boot sector

[Lopez, A.]

Hello!
I am new to linux and I am trying to install the Ubuntu distribution on my notebook (ACER 2012 WLMI).
I want to have a dual boot system, but it simply does not work.
At the beginning the notebook came with 2 fat32 partitions, a 50 GB partition (Windows XP) and an additional 10 GB fat32 partition.
What I did is to resize the 50GB into a 40GB partition and a 10 GB partition in order to install ubuntu in the later. 

Here it is my partitioning scheme now:
#1 primary 39.9 GB fat32 (windows XP)
#5 logical 2.6 GB ext3 /
#6 logical 6.4 GB ext3 /home
#7 logical 337.2 MB swap
#2 primary 10.4 GB fat32
#3 primary 8.2 MB fat16
unusable 98.7 MB unusable

The installer detected windows on another partition and grub was installed for dual boot.
However, after reboot, the 'invalid boot sector' message appears and no GRUB menu pops up.
Perhaps I did something stupid! Please tell me!!
Any idea that could solve the problem would be greatly appreciated!!!!!
Alex