Pleas use the URL in my signature and have a look how to post a question in a better way. As for your question. According to the data that you gave us (id with no gaps, need the average of 2 records) you can do it this way:

create table Demo (id int, Time datetime, value numeric(3,1))

go

insert into demo (id, time, value)

select 1, '10:00:00', 4

union select 2, '10:05:00', 6

union select 3, '10:10:00', 3

union select 4, '10:15:00', 7

union select 5, '10:20:00', 8

union select 6, '10:25:00', 2

union select 7, '10:30:00', 4

go

select a.id, a.time, a.value, (a.value + isnull(b.value, a.value))/2 as AvgCurrentAndPref

from demo as a left join demo as B on a.id = b.id+1

go

drop table demo

Notice that if you have gaps in the ID column, you can use common table expression with ranking functions instead of the ID column.

Adi