How much should I worry about the W3 Validator?

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Grey Alien

Hi, if I run my site through the w3 validator I have lots of errors:

http://validator.w3.org/check?uri=http%3A%2F%2Fwww.greyaliengames.com%2Findex.php&charset=%28detect+automatically%29&doctype=Inline

However, apart from the <td background> not being a valid attribute error (!weird!) all the rest are with code that I've pasted in from other sources such as google adsense, flash and reflexive affiliate links. It moans about varous tags in the pasted code, should I worry about it? Maybe it can be commented out but still work or something? Feel free to view my source if you need to.

I know I'm missing some Alt tags, but I'm not worried about them right now.

Also my mailto in the footer seems to cause it to flip out. weird.

Well I'm still a web noob so I guess it's to be expected.

Any help which will expand my feeble knowledge is appreciated thanks!