The remote computer requires Network Level Authentication, which your computer does not support. For assistance, contact your system administrator or technical support

To resolve this problem follow this method

Method 1:- When you have problem to Remote Desktop Connection from windows xp to server 2008. First of all install windows xp sp3. Now download and install this fix it and restart your computer. Your problem will resolve.you can download fix form link 1 or link 2.

Download
Link 1
Link 2 

Method 2:- Manually enable Network level Authentication form registry.

1.       Install windows xp sp3

2.        Click Start -> Run, type regedit, and then press ENTER.

3.       In the navigation pane, locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa

4.        In the details pane, right-click Security Packages, and then click Modify.

5.       In the Value data box, type tspkg. Leave any data that is specific to other SSPs, and then click OK.

6.       In the navigation pane, locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders

7.       In the details pane, right-click SecurityProviders, and then click Modify.

8.       In the Value data box, type credssp.dll. Leave any data that is specific to other SSPs, and then click OK.

9.       Close registry and restart your computer and try now it will work.
 

      Method 3:- In this method you will change remote desktop setting on server 2008. select my computer and right click and go to properties. Select remote setting form tasks.

 

      Change remote desktop setting and select “Allow connections form computers running any version of Remote Desktop (less secure)". you can also select users which need remote desktop access.

 

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