Vista is a dog. It's hard to say what the problem might be but perhaps try some of the following . .
1. Start -> Control Panel -> System -> Advanced System Settings -> Performance Settings -> select Adjust for best Performance
2. Tidy the Desktop so it isn't full of icons and remove any widgets that might be on the Desktop
3. Click the Start button -> type msconfig on the box and hit Enter -> chose the Selective Startup radio button and then click the Startup Tab -> you can pretty much untick everything in here except things related to antivirus (then reboot)
4. Perhaps the Antivirus is causing a problem, try turning that off as a test to see if there is any difference (if you needed to change AV program then you could uninstall that current one and install Microsoft Security Essentials, which is good and free)
5. You should update the Antivirus protection and run a full system scan. Also install superantispyware and run that, then disable or uninstall it.
6. I would (i always do) turn off System Restore in Control Panel -> System -> System Protection -> Highlight the C: drive -> Configure -> Turn Off . . note: this will delete any restore points
7. Check that there is no shortage of disk space, free some up if things are tight. Do Start -> Computer -> right-click on the C: drive -> Properties . .
I would turn off the option to have 'files indexed' (under General tab) and under the Tools tab I'd do an Error Check and a Defrag