After upgrading from XP Pro to Vista, my computer is incredibly slow, unresponsive and crashes constantly. Pretty much anything I do it becomes slow. It's becomes unresponsive every time I switch between tasks and tabs. I also noticed very low performance with score 1.0. The graphics score is 4.5, which isn't bad. I recently installed an ATI Readon 5500. So, that can't be the issue. The ram, 512mb, which is the minimum requirement for Windows Vista, and what does my computer need to make it run properly?

Specs

Dell OptiPlex GX270 SMT

Case: Small Mini Tower (SMT)  
Processor: P4 2.8GHz  
Memory: 512mb RAM  
Hard Drive: 40GB  
Optical Drive: CD-ROM  
Video: ATI Radeon HD 5500

Operating System: Windows XP Professional  SP3