Yes, the issue of installing applications slowing down the system is an interesting one. I remember seeing a report recently where they'd tested the difference in boot time after installing popular Windows applications, as well as some of the additional core components like DirectX, .Net runtime etc, and it showed that each one added to the boot time slightly, some much more so than others. Obviously things like Norton AntiVirus increased the boot time by a lot, as they're actually loading stuff on bootup (that's why I don't use any realtime virus/malware protection, as it just bogs down your system and you don't really need it if you know how to avoid that kind of stuff anyway), but it's interesting that programs which don't load anything on startup also seem to have an (albeit minor) impact. I guess it's just because they make the Registry larger, and that is loaded on startup. My PC (see below for specs) actually takes about 70 seconds to boot now (from power button to full desktop), which is about the same as 10.4.9 on the G3 (only 70 seconds to boot a cutting-edge machine on an 8.5 year old free computer...truly awesome

), although it used to take only about 40 seconds so I think it just needs a reinstall as the Windows installation is 2.5 years old now. Certainly fresh installations do boot very quickly, so I think a large Registry is probably the problem - I do have a lot of applications and games installed. OS X not having a Registry-like system probably helps with boot time.
As for Vista, it is meant to be faster, at least if you have a modern system (on a system near the minimum specs it will be slower because it's heavier). This is because it's the first major reworking of Windows NT since version 4.0, so is now optimised for the computers of today eg the memory management is designed for PCs with 1 GB Ram, as opposed to 16 MB. For example, you'll have seen people complaining how large a proportion of their Ram is being used when they start Vista up, but actually this reflects how it now exploits all of your Ram by doing things like loading frequently-used programs into it (SuperFetch). If the Ram is then needed by a program, Vista will release some of it. USB keys can also be used as SuperFetch space (ReadyBoost), which is meant to make things faster.
It's more stable as far as that drivers are no longer meant to be able to crash the system (most serious crashes on previous versions were caused by drivers, or bad hardware) - a lot of drivers (I know graphics drivers, often a big problem, definitely have) have been moved into userspace from kernelspace, so they simply reload if they crash. Apart from that, XP was pretty stable so there wasn't a lot of room for improvement.
It still has the Registry though, so I assume the same issue of large Registries will still exist.
I haven't installed it outside of a virtual machine yet though, so will give you my impressions once I have it up and running, if anyone's interested.
Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)