The MSJVM is a dead product; it's no longer shipped with Windows yet it can be found on many PCs, maybe even yours. That's because it was included in every version of Windows from 95 through to XP SP1a.
I recommend that you disable MSJVM because it's a known security risk. There have numerous MSJVM security problems in the past and more will surely follow.
MSJVM is used to run Java programs (applets). Such programs are quite common, both as stand alone programs like the popular BitTorrent client Azureus. Java is also commonly used to support advanced web site features.
But you don't need the Microsoft Java engine to run Java, you can use the Sun Java engine instead. Well, almost. A small number of Java applications utilize non-standard features in Microsoft's version of Java and may require the MS product to run. Such applications are rare outside corporate environments; I've not encountered a single example in the last 12 months.
Sun Java not only replaces MSJVM, it's a safer, better product and unlike the MSJVM is being actively developed and enhanced. That why I recommend you use it to replace the MSJVM.
The MSJVM can be removed or disabled. Disabling is the preferred option; it's less problem prone, is simple and quite effective. Here's how you do it:
Step 1: Find Out Whether You Have the MSJVM
Click the Start button and select "Run"
Type "command" into the run box (with no quotes) and hit Enter.
Type "jview" into the command window and hit Enter.
If you don't have MSJVM installed you'll get a message very similar to this:
"'jview' is not recognized as an internal or external command, operable program or batch file."
If that's what you see then you are finished; you don't have MSJVM on your PC. You are safe.
If you do have MSJVM you'll get a message like this:
"Microsoft (R) Command-line Loader for Java Version 5.00.3805 Copyright (C) Microsoft Corp 1996-2000. All rights reserved."
followed by about 20 lines of usage and options information. If that's what you see, go on to step 2.
Step 2 Install Sun Java JRE
Go to this page: http://www.java.com/en/download/manual.jsp.
Click the second button (manual installation) and download the file to your PC. This is a big 16MB download so allow plenty of time if you have a slow connection.
The downloaded file will be called jre-1_5_0_09-windows-i586- p-s.exe or something similar. Locate this file and double click it to install Sun Java on your PC.
Reboot your PC if requested.
Step 3: Disable the MSJVM and Ensure Sun Java is Enabled
Open Internet Explorer.
Select Tools/Internet Options From the toolbar.
Click the Advanced Tab.
Scroll down to the section called "Java (Sun)" and ensure that there are ticks in all check boxes within this section.
Immediately below will be a section called "Microsoft VM." Remove all ticks in all check boxes within this section.
Click "Apply" then "OK".
You are now finished; MSJVM has been disabled and replace by Sun Java JRE.
Removing the MSJVM
I don't recommend removal as disabling provides adequate protection for most users. However, if you want the absolute highest level of security and you are an experienced user then you may want to consider removing MSJVM entirely. Note that this is an irreversible process so it carries a few minor risks. That's why Microsoft pulled their free MSVJM removal tool from public download. To get the tool you'll need to write to Microsoft's support section where will make you jump through a few hoops to get it. Full details here: http://support.microsoft.com/kb/826878#kb4
Thankfully, the MVPS.org site offers the same MS removal tool for direct download (301KB) with no hoops: http://www.mvps.org/marksxp/Downloads/WinXP/Java/unmsjvm.exe
It's also quite possible to remove the MSJVM manually. You can find full instructions here:
If you have some unanswered question about the MSVJM you can consult this comprehensive Microsoft FAQ: http://www.microsoft.com/mscorp/java/faq.asp