The main criteria for running Windows vista narrows down to the graphics. Within a 45k budget, you can get a Core Duo powered laptop based on GMA950. The minimum system requirements to run the basic Windows Vista edition would be something on the lines of a pentium M /core duo processor, 1 Gb of System Memory (if you are using onboard GMA950 the memory is shared for graphics, hence 1Gb , preferrably two 512MB in dual channel), nearly 15-20Gb free space on your hard disk and a minimum of 128Mb video graphics. You wont get the eye candy of the Windows Aero Interface but rest assured it would run vista albeit the basic version.
You should look out for the Dell Inspiron 6400 or 640m. they are the best choices for around 50k to get a Vista Ready laptop.