I am fairly new to Ubuntu (and to this forum as well) so please allow me to be clueless. I recently installed the precise 12.04 LTS (alongside windows) in one of my older desktops (asrock chipset mobo, P4 2.4GHz CPU, onboard intel graphics, 1 GHz ram). I am using the Gnome classic DE and had set the OS to automatically update. Yesterday the OS downloaded and installed updates as usual and asked for a reboot...and I complied...however ubuntu refused to boot since then...the grub menu would appear (without the timeout countdown which was earlier set at 5s) and on pressing enter a purple screen would appear momentarily followed by a black screen with a blinking cursor at the left upper corner of the screen. I could boot to the windows installation though and could also boot using a live usb. I googled without any luck...finally I partially solved the problem (serendipitously?) by finding that the linux version in the grub menu had changed to 3.5.0-26-generic from the previous 3.5.0-25-generic...I located the previous linux version in the "previous linux installations" item in the grub menu and booted to that without any issues.
My question now is this: how can I make the 3.5.0-25-generic my default version? also can I safely remove the latest version of linux i.e., 3.5.0-26, if so how? Also how can I prevent ubuntu from updating to this latest version during future automatic updates (as it seems that the latest version is not compatible with hardware (graphics etc) of my desktop and the 3.5.0-25 seems to be stable on my pc and is kind of working for me now)?
Thanks in advance