Windows Server 2008 (R1) Service Pack 1
Windows Server 2008 (R1) SP1
W2K8 (R1) Service Pack 1
W2K8 (R1) SP1

Note: You should check http://support.microsoft.com for the latest version of the different files.
Note 2:  Windows Server 2008 (R1) Service Pack 1 regular support has ended.

List of Microsoft Network Load Balancing (NLB, a.k.a. NLBS) cluster related post Service Pack 1 for Windows Server 2008 as of Mar., 2012:

Originally posted Jun. 2009.

Preferred:
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Please look into installing "Service Pack 2"
Windows Server 2008 Service Pack 2 and Windows Vista Service Pack 2 - Five Language Standalone (KB948465)
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=16468
 
and
 
Check for post SP2 hotfixes such as in:

List of NLB related hotfixes post Service Pack 2 for Windows Server 2008
http://blogs.technet.com/b/yongrhee/archive/2009/10/11/list-of-nlb-related-hotfixes-post-service-pack-2-for-windows-server-2008.aspx


Alternate:
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972183 Users may experience periodic connection loss for applications that are running in a Windows Server 2008 NLB cluster using an IPsec connection
http://support.microsoft.com/?id=972183

Updates:
Nlb.sys

Supersede(s):
960916 Network Load Balancing (NLB) clients cannot connect to the Windows Server 2008 NLB cluster by using the virtual IP address when NLB is running in multicast mode
953828 Windows Server 2008 Hyper-V virtual machines generate a Stop error when NLB is configured or when the NLB cluster does not converge as expected

973780 Some TCP connections between an NLB server that is running Windows Server 2008 and its clients are broken after the Port Scalability feature is enabled on the NLB server
http://support.microsoft.com/?id=973780

Update(s):
Rpcrt4.dll

963042 IPv6 network connectivity problems occur on a Windows Server 2008-based computer when the Network Load Balancing feature is enabled
http://support.microsoft.com/?id=963042

Update(s):
Nlb-unattend.exe
Nlbcfg.dll
Nlbmigplugin.dll
Nlbmprov.dll
Nlbmprov.mof
Nlbmprovuninstall.mof
Nlbtrace.mof
Wlbsctrl.dll
Wlbsprov.dll
Wlbsprov.mof
Wlbsprovuninstall.mof

Supersede(s):
959493 The WMI Provider Host program (Wmiprvse.exe) may crash on a Windows Server 2008-based computer that has the NLB feature installed

949429 The virtual IP address of a Windows Server 2008 NLB cluster is bound to the NetBIOS host name of a particular server or of multiple servers
http://support.microsoft.com/?id=949429

Update(s):
Netbt.sys  6.0.6001.22119  
Netbtugc.exe  6.0.6001.22119