AJAX not working

Previously I was using php fastcgi but due to some page loading problems I had to shift to php5 (normal). the time when I was on fastcgi every thing was ok but once I moved to php5, all my ajax related plugins stopped working. For example my wp-poll plugin returns 404 for admin-ajax file, wp-dtree doesn’t load,etc I have tried disabling all pluging as well checked with default theme with all plugins disabled, but still no luck.

Is there any way to fix this,
Or otherwise
Is there any way so that I do a fresh install on a new database and can import ally my post , categories, comments, etc (alongwith plugins data) into it

Thanks