As long as you have the 32 bit version of Vista, click on Start, then type "Run" in the Search line, then type the command below in the "Run" line:

regsvr32 jscript.dll

Press the "Enter" key.

As long as there are no error messages, it's now registered. (Full instructions for running such from a command line in both 32 and 64 bit versions is in the link below:

As far as jscript.dll not being registered, you might not notice it much unless you use a lot of JavaScript scripting on webpages or programs on your computer. If you've got scripting disabled on your browser, you might not notice it all.

Hope this helps.