Yes, that is exactly what the Airport Express is designed to do.
One thing to keep in mind is, that if your speakers only have one 3.5" jack input, then you may need a 3.5" splitter to connect both the Airport Express and PC to the speakers.
Why connect the PC to the speakers via cable, if the iPad can Conner wirelessly, you ask. Apple restricted the AirPlay feature, so iTunes is the only desktop application, which can stream music wirelessly to the speakers. There is an app named AirFoil, which let's any program stream music to the speakers, but it is limited to one program at a time. In contrast, all iPad apps can stream their music wirelessly.