Code to emulate what excel does in cells

This is simply a translation of the recursive rules indicated:

Block[
  {step = 0.001, p = 1000},
  NestList[
    Apply[{#1 + step, ((#2 + 273.15) - p*(#1 + step)) - 273.15} &], 
    {0, 50}, 
    6
  ]
]

(* Out:
{{0, 50}, {0.001, 49.}, {0.002, 47.}, {0.003, 44.}, 
 {0.004, 40.}, {0.005, 35.}, {0.006, 29.}}
*)

Is this close enough?

TableForm[
{{"step",b1=0.001},
{"p",b2=1000},
{" "," "},
{"Time(s)", "T[C]"},
{a6=0,b6=50},
{a7=a6+b1,b7=((b6+273.15)-b2*a7)-273.15},
{a8=a7+b1,b8=((b7+273.15)-b2*a8)-273.15},
{a9=a8+b1,b9=((b8+273.15)-b2*a9)-273.15},
{a10=a9+b1,b10=((b9+273.15)-b2*a10)-273.15},
{a11=a10+b1,b11=((b10+273.15)-b2*a11)-273.15},
{a12=a11+b1,b12=((b11+273.15)-b2*a12)-273.15}}]

or this

TableForm[Join[{{"step",b[1]=0.001},
{"p",b[2]=1000},
{" "," "},
{"Time(s)", "T[C]"},
{a[6]=0,b[6]=50}},
Table[{a[i]=a[i-1]+b[1],b[i]=((b[i-1]+273.15)-b[2]*a[i])-273.15},{i,7,20}]]]

but I suspect that you can easily keep adding more requirements or writing more complicated cross referencing formulas in your spread sheet to break any answer.