thank you all, here some details

the app was in a normal computer with winXp Pro, I took a copy of it and put it on a WinServer2008 and then I installed the VDFClient and the PervasiveSql.

a company (that has to do with nets stuff) studied this case and told me that both the VDF/Pervasive combination is a bit heavy a take a lot of resources in the net and was causing the sesions to hang ( ?... ). I not sure about this but they suggested to put an Adsl of 2 megas for upload/download.

BTW Karl, The server has installed an antivirus called something like "claun free internet antivirus", I do not know if this can be causing problems but they resist to use another antivirus.

Thank you all for your help