Quick, grab a Service Manual from http://tim.id.au/laptops/hp/hp%20540%20541.pdf
OK, notice on page 15 the LCD is the entire FRU #500004-001 and does not supply just a LCD panel. You might find LCD panels in the USA on Ebay and such but I can't guess where you are.
The symptom sounds like a bruised LCD. Unless you can obtain a panel and install it for under 100 bucks I would not repair this. Your choice of course.
Actually my laptop( HP 540 and OS is windows 7)
screen has generated 2 dark patches ..one appeared 3 month back and it is quite big in size say 4-5 inches ...the other dark patch appeared hardly 20-25 days before and it is growing slowly ....so what might be the cause and what are remedies ....Do I need screen replacement ....I fear it may cost too much