RAM problem (urgent)

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[SIZE=13px][FONT=verdana]Computer Specs [/FONT][/SIZE][SIZE=13px][FONT=verdana][/FONT][/SIZE]
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[SIZE=13px][FONT=verdana]Dell XPS 400[/FONT][/SIZE][SIZE=13px][FONT=verdana][/FONT][/SIZE]
[SIZE=13px][FONT=verdana]Specifications: ([/FONT][/SIZE][SIZE=13px][FONT=verdana]http://support.dell.com/support/edoc…0/sm/specs.htm[/FONT][/SIZE][SIZE=13px][FONT=verdana] )[/FONT][/SIZE][SIZE=13px][FONT=verdana][/FONT][/SIZE]
[SIZE=13px][FONT=verdana]Radeon HD 4800[/FONT][/SIZE][SIZE=13px][FONT=verdana][/FONT][/SIZE]
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[SIZE=13px][FONT=verdana]I am visiting a friend in west virginia and he purchased a 1gb stick of ram with the incorrect pins that could fit his mother board. I removed his elder ram stick ( as all of his 4 slots were full with 512 MB sticks ) and noticed it was the wrong size. I reinserted the original RAM . I loaded windows and windows no longer was showing 2 gigs of ram ( the starting point of RAM available ) and now only shows 1.5 GB , which indicates ( obviously ) that the computer is not detecting the RAM stick i removed and reinserted. I tripple checked that it is in right and I just cannot figure out why it is acting like this. I am in desperation to fix the issue because now I have slowed down the person I am visiting’s computer ( as his Windows XP takes up 1600-1800 MB of ram in casual use ; so now it is freezing ). If someone could give me several routes to advice, please do so. I will be checking the forum frequently as I am trying to fix this as soon as possible.[/FONT][/SIZE]