Many times it is several little things that add up to a slow PC. Strip down your startup, and taskstart to a minimum. To much e-mail saved or unread will slow your computer down, and then there is the obivous spyware, adware, and virus attack. I am on Time-Warner Roadrunner and they offer a CA security at no additional cost. It works.
There are many Windows Services that start on boot-up that are not needed in a home computer, get a list of the services you can disable.
Finally, clear your browser of temp files and such. Keep your harddrive defragged. Hope that helps
OK, have to say that the MAIN problem you have is Vista64 Ultimate. 2GB of memory is not enough with this OS, you should double that. Or, ideally, upgrade to Win 7. I have Vista on only 1 of my six machines, and after seeing the resources it requires, and how SLOW (even booting up), it is, I refused to upgrade any of the other machines. They are all still on XP. Seriously, the previous replies were also good ideas, you should always make sure that Cache is cleaned, the disk is defragged, and your registry is not loaded with useless pointers that have not been removed properly when you added or deleted software. Additionally, your startup applications should be examined frequently to make sure that ONLY those necessary for what you are actually running is resident. Many software packages will start background "update" tasks, that will prompt you each time a minor update to the software has been published, I would recommend that you disable these as they take up memory, and manually checking for updates works just fine. This goes for the Windows update task as well.
It took me a while to learn Vista (32), but once I did, it really is not that bad. Just have to do every thing backwards from W2K or XP. Maintanance is the answer, and a lot of computer "service wanna be" has learned this. They are out there cleaning IE Caches for $75 a pop.
I have had good success working on slow computers by utilizing general maintenance. Clean out temp folders including IE, not letting Outlook mail folders to fill, and keep the spy ware off the system.
I go to the registries directly,primarily the 'run' key under Windows. If it ain't supposed be there the first time I comment out the data line, and if that doesn't seem to affect anything I will delete it later. Anti virus, and defrag seems to get the job done. I run Vista/3.8Ghz/2 Gig Ram.
All of the above is good general advice but i still believe your biggest problem is RAM. for vista ultimate you should have 3GB if DDR3 or 4GB if DDR2 at least.
What kind of processor do you have? And also what video card are you using? If you have visual enhancements turned on (such as Aero) you will need a powerful 3D graphics card. Be sure to choose a basic windows theme and turn-off any animations and visual enhancements.
Mostly I agree with the above but don't run a registry cleaner. Many, if not most, are junk and some can - and will - severely damage your PC.
I would suggest some software like CCleaner - not the registry part, just for cleaning temp files, and defragging after that. Then get some more RAM, but make sure it is compatible with what you have already!
Finally, if you have "some games" do you have a separate video card - and a good one?
Not to step on the toes of other posters here, but consider this. There is a REASON Microsoft released and pushed out sales for Windows 7 not even 2 years after Vista. Hate to say it, but from what I have seen, Vista has become the new WinME of todays age.
Maintenance is a MUST with stripped down start-up, clean defragged drives, etc sure. But, here is what I cant explain. Identical architecture workstations side by side. Both 64 bit machines. Windows 7 64-bit running at nearly 50% less resource strain on 64bit bus architecture for tasks that Vista 64bit runs those same tasks. I have seen it. I tell people now who have not yet upgraded from XP when they ask VIsta or 7 to forget that Vista even exists. There are too many issues with it and Microsoft seems to be throwing the full brunt of their development and support behind 7 now. I really do think Vista was more or less of an "ooops" OS that 7 corrects. Like how Win2000 was much more stable than WinME and several homes ran the 2000 "business" environment over the "home" solution of ME