installing multiple copies of the same software doesnt work for all softwares because during installation the software, it might detect an existing installation and refuse to install another copy.
so you may delete the registry enteries of the first installation and install a new copy.
{NOTE: editing the registry is dangerous under the hands of the ignorant}
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but wats the use, this is almost like a "copy and paste" method.
in this method, you copy the entire program's folder and paste it elsewhere and run the new instance of the program from that location.
{NOTE: this doesnt work for all softwares}
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but why do the two methods above wen the next method will save you all the trouble.
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now... the best way is to right-click on the program exe and select "run as..."
now select a different user, under which the new instance of the program should run.
for eg. {considering you have "Guest" account enabled and the you still have the default administrator account, normally named "Administrator"}
you may run one instance under "Guest" account, another under "Administrator" account and one under your current account.
you can do this for as many number of accounts there are, provided you have acces to them.
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the last method always works...