Well since your laptop is from around 1999 - 2001 (I'm guessing), you will not be able to install Windows Vista or Windows 7. That means Windows XP is going to be the most recent OS that you can install. Correct me if I am wrong about the age of your laptop.
You can purchase an OEM version for Win XP Home w/ Service Pack2 for $108 + shipping & tax:
http/www.ecrater.com/p/8956157/microsoft-windows-xp-home-sp2
OEM (original equipment manufacturer) is generally meant for companies (like Dell) to buy and install on PCs that they sell and those companies are responsible for providing you with technical support. Additionally, if you want to install the OS on another PC you will need to contact Microsoft to release to product key so that you can use it again.
Here a full retail version for $144. The difference is that you do not need to ask Microsoft's permission to install on a newer laptop if you decide to "upgrade". I prefer full retail myself since I don't want to be bothered calling Microsoft every time I upgrade.
http
/www.computersystems2go.com/ProductDetails.asp?ProductCode=MSF.WinXPHome.C&Click=2585