I own a Dell Inspiron 1525, it has a hard drivehttp://www.amazon.com/gp/aw/d/B005T3GRLY/ref=mp_s_a_1_6?qid=1440976907&sr=1-6&pi=AC_SX110_SY165_QL70 am unsure what OS to use my budget is infinit and I have the discs for . xp, 7, or vista.Strange question. Use the latest OS you you have a legal license for.
Not sure how to advise you.  You say you have the disks for XP, 7 & Vista.  Between those three I would choose 7.  You then say your budget is INFINITE so in that case you may want to consider buying a copy of Win 10 and doing a clean install.  You have ENOUGH disk space to do something like dual boot  7 and 10.  Another option would be to install 7 then do the free upgrade to 10 to see if you like it, if not, roll it back to 7.

In order to get better advice, you need to tell us what you use your machine for.  Is it for gaming? Development?  Word processing? No need for him to purchase Win 10 if he has Win7 already...If you read what I said, he could also install 7 first and then do the upgrade but since he has an infinite budget, he might want to forgo the upgrade and buy Win 10 out RIGHT thus no need to do an upgrade. Quote from: strollin on August 31, 2015, 10:36:18 AM

since he has an infinite budget, he might want to forgo the upgrade and buy Win 10 out right thus no need to do an upgrade.

You can do a fresh install of free Windows 10 onto a machine that has a qualifying OS install. No need to spend money.