I want to change the server from a Windows 2000 Server to a Windows Server 2008 environment, so I'd like to know how can I migrate my TWiki too. Anyone?
Sure. You install TWiki on the new server, get it running, authenticating, etc., and follow the Manual Upgrade process published on twiki.org. You have to exercise some care to get the permissions correct (see the site), and to make sure that the new config file is correct.
There are a ton of possible glitches, which you will have to deal with as you go along, but nothing I can see to make it impossible. The biggest wrinkle I can see is if you are using server authentication rather than TWiki authentication. If so, I recommend that you switch to TWiki authentication, move the site, and decide if you want Windows/AD authentication later.
As a former TWiki administrator, currently using Foswiki instead, I'd have to agree with SvenDowideit if you are willing to go from IIS to Apache - Foswiki really is very, very compatible with TWiki so it's more of an upgrade than switching to a different product. Foswiki is much easier to install on Windows (just checked the TWiki Windows options and it seems you have to pay for an easy install for Windows, which also involves using VMware); plus, Foswiki is actively maintained and unlike TWiki there is no 'commercial edition' with all the nice features.
If you want to keep IIS (understandably): using TWiki or Foswiki on IIS is always going to be painful compared to Apache, due to the way IIS works, but this Foswiki IIS install page is at least fairly up to date. Use Strawberry Perl, it installs very easily and is just like "Linux Perl" in that CPAN modules will work better.
Unless you want the paid-for TWiki features in the commercial TWiki edition, I'd suggest that upgrading to Foswiki is well worth considering. The TWiki-Foswiki split was not at all amicable and 95% of the active developers went to Foswiki, so that's where the community is these days.
First and foremost, do you need to use IIS, or can you turn it off and run Apache?
If you can use Apache (Foswiki/TWiki run much faster under Apache - as Perl CGI support is somewhat immature on IIS even now), then the easiest way to get going, is to use the Windows Installer that I wrote - see http://foswiki.org/Download/DownloadFoswiki
The installer contains Foswiki, Apache and Strawberry Perl and sets itself up. From there, all you'll need to do is copy over the changes webs (both data and pub) and migrate the .htpasswd file...
(Yes, I'm spruiking Foswiki, the TWiki fork, because I know how much work we've put into building more tests and fixing bugs since the non-amicable split with the TWiki Inc company late in 2008)
I too would recommend using the LDAPContrib - the one on foswiki has had several years more development too.