Hi Jupiter4
Have a look at the VST site links on this site, http://www.funk-station.co.uk/links.htm in particular Synth Edit & FXPansion (Which include GURU) all these can be used alone on a computer or integrated in to any keyboard that accepts VST Instruments.
Regarding Windows stability, bear in mind that the Wersi instruments were developed in the late 90s, and were released on to the market in 2000 with OAS 2 and Windows 98, this was upgraded to Windows XP in 2002 when OAS 4 was released, I can also report that over the 5 or 6 years that the OAS instruments have been on the market, there has never been any problems with the Windows operating systems, (Exceptions are when people who think they know better then Wersi, alter and mess up the operating system) this is because Wersi modify the operating system to give optimal performance and stability with the OAS software, so you do not have to worry about it.
Like any software, sometimes bugs are found in the OAS system but as soon as Wersi are informed, then they get straight on the job to sort it out, and then offer a free update CD. (In the early days support was not as good as it could be, but with the new management team there is now very little problem)
Regarding Wersi itself, they have been in business since 1969, although there have been management changes over the years, as with a lot of firms.

Bill