What version of Win98 is installed? You can check to see if you can add English in the options, but you will need the CD for that. There will be an add additional languages button there, select that and follow the prompts.
This is usually not recommended, but you could install an English version over the old one, putting the system files in a directory other than WINDOWS... but having 2 system occupying one drive can be extremely confusing for the programs files and document files.
Think of the programs that you have installed, and whether you can reinstall them. If it is not too much to do, then I recommend that you backup your data, format disk then install windows. If the software is not available then overwrite the current windows installation by reinstalling windows in the same old directory, hoping all programs will continue to run well.
Exactly what software is installed that you cannot replace? Since the system is in Spanish and you don't speak it, how did you use it and install the software? It is in your best bet to find replacement installation media for the software you are afraid to lose and back up any personal data in the program data and documents folders. And then make sure you have all your drivers from the PC manufacturer on hand.
Installing on top of your current install but choosing a folder other than windows will let you be able to keep what you have until you can fix it. If you install over the existing installation without reformatting the hard drive, you will likely have problems. But anything other than a fresh install will cause you headaches somewhere down the road anyways.
It will be best to get everything together then reformat and reinstall.