Don't worry, very probably only the bootloader was overwritten in the MBR. Fix is to re-install your bootloader (Grub, I assume). For that you'll need a bootable system, for example a live-cd distribution--Ubuntu live cd would do fine. LVM does mean you'll have to do some steps more than without it, but it shouldn't be too difficult to activate the volume groups using the live-cd, and then use it to re-install Grub. For precise instructions, see for example this article
which deals with the exact same issue. Note that it shouldn't matter which distribution (or OS for that matter) you use to recover the system, because you're merely reinstalling a bootloader that can boot that system. In fact you could probably use the Windows bootloader to do that as well, if you wanted to. If you have Ubuntu live cd available, that should be good: the requirements are that there are tools available that you need (for LVM and Grub installation), and that it can recognize your hardware etc.