I have never seen a false positive with WPA or WGA. It is always where the software hasn't been activated, and is outside of its activation period. Of course its possible someone has a licence, but has run a crack for whatever reason, but this should be an unlikely scenario. And most people capable of installing XP should know better.
I have heard of false positives, although I'm unsure of the details such as whether the correct key was used etc.
Funnily enough I bought a new 160Gb HD for my Dell laptop, then discovered that the OS installation CD that came with it is gubbed (shows a general protection fault a few seconds after booting). Brilliant! Couldn't find a Dell OEM installation CD for download so I hastily downloaded a standard WinXP SP2 ISO using my Linux desktop - oh the irony lol, and installed with that. But of course it wouldn't accept my OEM CD Key so a quick search of google groups (usenet archives) came up with more keys than you could ever wish for. First one worked fine. A few days later I managed to get a hold of a Dell OEM CD which did work OK so I started again and reinstalled "legitimately". Unfortunately it was only an SP1 CD so I had to spend ages getting SP2 and all the subsequent updates. Funnily enough it didn't even ask for my CD Key this time, apparently this is normal for Dell systems, maybe it can read something from the BIOS or whatever. Didn't have to activate either, and of course WGA isn't installed cos I won't allow it to be installed.
The cracks do not effectively work with WGA, most of them just hide the prompts.
It plays havoc if you are using imaging technology to deploy multiple identical machines, but in that case you should be looking at corporate versions available with Open Licencing, which handle it slighty differently (as most non legal XP users will know - no activation after install, but if you use one of the 2 well know keys, the service packs will detect this).
Couldn't tell you anything about the cracks, never seen any of em. CD Keys can be changed easily enough, there are a few workarounds that I've heard of (the classic one being to change a registry key to make windows become "unactivated", then use the activation wizard to change the key). So you then just go onto google groups and find a key that works with SP2! Yet another useless "solution" to the piracy problem.
MS have to do something to prevent piracy, WPA/WGA is a reasonable start.
But the guilty until proved innocent approach is not something I want to have any part of. If I have paid £200 for XP Pro (or more likely about £80-£100 for an OEM version bundled with the PC) then I do not see why I should have to repeatedly contact M$ to let them check that I've paid for the software. No other mainstream vendor does it, and they all have piracy problems.
It's akin to the government saying we have to do something to tackle the problem of drivers breaking the speed limit, so from next year every car will have a GPS device that calls home to your nearest police station to check you haven't broken any speed limits.
Both are illegal, but I don't agree with the principal of some "higher authority" periodically checking my property to ensure I'm adhering to their rules.
