Uninstall - right away - both Avast and MacKeeper. Although the free version of avast gets some good scores from Thomas Reed's av detection list, it seems as if it's causing you problems. And adding MacKeeper on top of it is just making things worse (see Thomas' Beware MacKeeper).
I would bet that you would be able to start up in Safe Mode - holding down the Shift key whilst booting...
Contact Apple and order replacement installation disks. Have your MBP serial number ready which you should be able to find on the bottom.
Get rid of Avast and MacKeeper. These programs do more damage than good. Best way to proceed is to erase the HDD and then reinstall the OS when you get your disks.
In the interim look at this Apple support article:
Ive tried getting in via safe mode by holding shift and although this worked at first, it has obviously been getting worse and this does not work at all, i have tried on and off constantly for the past 3 hours by holding shift with no success.
How do i remove avast and mackeeper if i cant boot it up? The same problem as trying to use my time machine?
It looks like you both suggest i need new discs? At what stage do i need to start looking at a new computer as i'm guessing replacement discs arent cheap.
I was hoping that i can eventually get on in safe mode, use time machine and go back a month, hoping this would sort the problem out.
Thanks for your replies, they are very much appreciated.
I think that OGLETHORPE is writing about a replacement set of DVD's that would have come with your computer. About $20 last time I checked. You could boot from the installation disc (maybe) and restore your MBP to it's original state (which you may just have to do - good thing that you have backups).
When you get your replacement disc(s) just boot from the installation disc, use Disk Utility to erase your drive (format as "Mac OS Extended (Journaled)" with a single GUID partition, and then use your Time Machine backup to restore your system as it was before you installed avast and MacKeeper.
Call 1-800-MYAPPLE to order the disc(s). As OGLETHORPE says, you'll need your serial number as the disc(s) are machine-specific.