I did it! I had done an extensive internet search for figuring out how to connect my old Valsat 2008 to my laptop PC and hadn't found anything. Some people even said it was impossible. Well, it wasn't that difficult after all. For all those people out there looking for a solution, here's how I did it:
The Valsat 2008 complies with the NMEA-183 standard. This standard recommends the talkers to use the RS-422 protocol. In practice, most modern GPS devices don't follow this recommendation and use the well-known RS-232 protocol. For this reason, interfacing these devices to a PC is straightforward connecting them to the 9-pin RS-232 serial port. For connecting my Valsat 2008 to my laptop I had to overcome two problems: 1) most modern-day laptops don't have any standard serial ports anymore, 2) the Valsat is one of the few GPS devices following the NMEA recommendation and using the RS-422 protocol. RS-422 is a differential system, having two signal lines, A+ and B- (instead of a single signal wire and a ground), making it almost immune to noise and interferences (where RS-232 is not). Connecting the GPS directly to a RS-232 port is no option, since it might permanently damage both PC and GPS device.
The solution to both of my problems was a 45?'? USB-to-RS-422 converter from EasySync, and a piece of cable with a female DB9 connector.
In the image below, one can see the connections on the back side of the GPS and the ones on the converter.
Data out+ goes to RxD+, Data out- to RxD-, Data in+ to TxD+ and Data in- to TxD-.
With this setup I have successfully interfaced the Valsat 2008 with the following programs: GPS Diag, VisualGPS, MaxSea, Offshore Navigator and ScanNav.
The USB-to-RS-422 converter drivers create a virtual COM port (COM5 in my case) which can be used as any other serial port. If necessary, in MaxSea the new COM port needs to be added (F4 -> "Add Instrument"). The Baud rate should be 4800 with 2 stop bits. Under "Advanced Settings" it is possible to select the NMEA sentences that will be interpreted and used. These should coincide with the ones sent by the Valsat (configurable in "Menu 5").
Against all expectations, I have also managed to upload Waypoints from the laptop to the Valsat. The Valsat's "Menu 4" should read:

OUT
NMEA183
NORMAL
IN
WAYPOINT
NORMAL

Select "MLR (Valsat series)" as the GPS device in MaxSea. Already existing Waypoints (same number) in the Valsat are overwritten. For the moment, I haven't managed to make MaxSea also send the Waypoint's name or comment. The latter is no problem at all with ScanNav, which knows the Valsat 2008 model.

Update May 18th 2007: in view of the number of times people have asked me for a copy of the Valsat 2008 User Manual and since this GPS receiver isn't anymore on the market, nor exists its manufacturer, I have decided to scan the manual and provide it here.