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[*]Can you have two toolbars installed at once?
[*]Does this affect the performance of the computer?
[*]Will it stuff up anything?
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The reason is I like some features of the Yahoo! toolbar and some of the toolbar
1. Yes you can.
2. Well, both toolbars will use some extra memory, so yes, but I don't think it will be noticable unless you have a very old PC.
3. It will decrease the size of your browsing space, since the toolbars will be placed at the top of the webpage you're viewing. I don't know about IE, but with firefox you can drag the toolbar to an unused spot on another toolbar so it won't take up any extra browsing space.
For example: I have google toolbar sitting next to firefox' Help menu instead of underneath the address bar.
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Two toolbars @ once
toolbar 
RE: Two toolbars @ once