My first question: what are the specs of your system? It's entirely possible (although I don't know this for sure, so someone else may need to confirm or deny) that 1.18 uses more memory than previous versions, so your machine might not be able to handle the extra load.
Secondly, did you alter your memory settings for your computer at all recently? Change the size of your swap file, add/remove memory, partition your hard drive...any of that stuff? Or are you running with the same set up as you were with 1.17?
Lastly, have you tried uninstalling and reinstalling, complete with the update? This would be a last resort type of solution, but sometimes stuff happens. And a complete uninstall/reinstall might do the trick.
Priest of the Church of Joe Wood
In memory of Nanee. 9/1/1993 - 5/16/2009. Rest in Peace, little girl.