You can use the target attribute of the <a> tag. Set it to "_blank" for a new window/tab. Note that it's generally not considered very "polite" to do this these days though, and it's actually invalid according to the XHTML strict doctype, so bear that in mind if it applies to you.
The first rule of Tautology Club is the first rule of Tautology Club.
As many others around here mention a lot, make sure you filter your input/output. Placing the value directly into a link with no filtering/sanitizing will make you open to xss attacks. Even if the input is yours you should sanitize it before outputting it to the html code. You should also watch using <b> tags, they're not deprecated yet but i have a feeling they soon will be. Use css styling instead.
And keep in mind, if you still want to be compliant, you can always attempt to make your own custom dtd, making it support target="_blank", i believe most browsers pop into standards mode still and it will validate. Though that may be more trouble than its worth.