Mistreated
My Documents, My Pictures - I've always hated them. And the same ire is directed at programs that insist on saving stuff there, or to the user application folders. Tell me, how much important data gets stored in C:Documents and SettingsUserNameApplication Dataetc and how many users never back up that area and lose it when they switch machines?
Lightroom's Windows Beta saves its library database into that area and gives no opportunity to save it elsewhere. That may change once it is can work with multiple libraries, and it will have to - its thumbs folder grows about 1 Gb per raw file. Add a thousand or so pictures and watch your system drive fill.
The fix is easy. Close Lightroom, and copy C:Documents and SettingsUserMy DocumentsMy PicturesLightroom folder to another location. Then delete the original folder in My Pictures and restart Lightroom. It will ask you to locate the library folder you want to work with. If you want to work with multiple folders, it's a variation on the same trick.