Are you using Nero? If so you could see if it's the media by switching off JustSpeed. How's this done?
If you go to this screen:
And click Advanced, and uncheck "JustSpeed enabled" you'll be able to write all media at 40x.
Now, if the writer is still writing at 25x, then you have some other problem. If it now writes at full speed you'll need to get some different media, or just run the drive with the JustSpeed feature disabled (that shouldn't be a problem with this particular drive and the discs you're using). Or you could try updating the firmware to the latest version.
If the speed is still slow, the most likely problem is your IDE port, either it's too slow (common on some cheaper motherboards) or DMA is not enabled (this is in the advanced properties of the IDE device driver).
Try running CD-Speed, which will have come with Nero, with an original pressed data CD of at least 60 minutes. See if the speed goes up to 30x/40x or whether it levels out at 20x -- if that's happening then the IDE drivers are not setup, or your hardware is suspect.