>>Thanks. Actually what I am looking for is preventoing the customer to take the installation disks/file and installing the software on multiple computers/laptops
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>You can't, and I don't think that's what you really want either. You don't want people to be able to run your program unless they have paid for it. Check out http://fox.wikis.com/wc.dll?Wiki~CopyProtection

Indeed -- and depending on the situation, copy protection may not be worth the trouble. It's really hard to come up with something that works 100% of the time and is bulletproof.

Enforcing single-install only by modifying the installation media might work in some cases, but could often be easily circumvented (e.g. with some floppy installations, you could get around this by simply copying the original disc and install using the copy -- if it didn't complain and installed OK, that meant you can simply copy the disc again and use the copy to install on another computer). Of course this method won't work with CD-ROM or DVD-ROM media.
One danger of a "roll-your-own" style of copy protection is it's often too simplistic -- and is often easily circumvented. Using a commercial library might be better -- but it might impose an extra cost (which may be a per-unit cost -- something you'll likely end up passing on to the customer -- something they probably won't appreciate). Online method may raise concerns (especially if this means your application periodically "calls home" -- something that could be seen as suspicious behavior). Hardware solution such as dongle may be less obtrusive -- though you're now faced with other problems (e.g. in the past, prior to the mid-1990s, it was common to have serial port or parallel port dongles -- they generally worked OK, save for the occasional problems when used in "pass-thru" mode -- but these went the way of the dodo when serial port and parallel port were replaced with USB ports). Currently there are USB dongles, but there's no guarantee you won't have problems with them later on due to changes in the hardware.
If the application is to be used on a network, this adds yet another problem. You can't simply "brand" the install copy since this prevents usage on more than one workstation...