Manual Origin of the Roman for 5:
[If you've forgotten what Roman Numerals look like, they're listed below.]
J. E. Sandys describes the origins of Roman numerals, in Latin Epigraphy. He says the original numeral was a line to stand for "1" and a representation of a hand that looks like a V to stand for "5". The X (=10) is two hands joined or one "v" atop a second "v" turned upside down. However plausible that sounds considering the human tendency to use our digits and hands for counting, there is an alternative explanation....
Etruscan and Greek Origin for Roman Numbers:
The X was like the Etruscan symbol for 10, the top half of which was adopted for 5. Ancient Chalcidic (Greek) symbols were added: one came to look like an L for 50 and the other was the Greek phi (Φ) that became the curved M (=1000) symbol that was sometimes written like our symbol for infinity.
Gradual Changes in the Roman Numerals:
To make the number 10,000 the phi was surrounded by an outside circle. A third circle made it 100,000. The right half of these symbols meant half, so half of the phi, a D meant 500, as used by Cicero. But towards the end of the Republic a horizontal bar above a numeral meant that number in the thousands, so a V with a bar on top meant 5,000, and a D with a bar on top meant 500,000.
More on the Higher Roman Numerals:
Somewhat surprisingly, Sandys says that M was not used for 1000 before the second century A.D., except before p for mile  milia passuum.
Sandys says the original symbol for 100 was probably a Chalcidic theta (Θ) and became a C. The fact that the word in Latin for 100, centum, began with a C made the symbol especially appropriate.
Sources on Roman Numerals:
In addition to J. E. Sandys' Latin Epigraphy, Mommsen is another source on the topic of Roman Numerals.
For more recent work on Roman numerals, see "The Origin of the Latin Numerals 1 to 1000," by
Paul Keyser. American Journal of Archaeology Vol. 92, No. 4 (Oct., 1988), pp. 529546.
Also see:
 Roman Numeral Dates  Conversion Guide

Roman Numeration
The numbers, how to decline them, and the hours of the day.
Explains how to figure out what a date in Roman numerals is in Arabic numerals. Includes an explanation of the inconsistencies.
List of Roman Numerals from I to CI:
1 I
2 II
3 III
4 IV
5 V
6 VI
7 VII
8 VIII
9 IX
10 X
11 XI
12 XII
13 XIII
14 XIV
15 XV
16 XVI
17 XVII
18 XVIII
19 XIX
20 XX
21 XXI
22 XXII
23 XXIII
24 XXIV
25 XXV
26 XXVI
27 XXVII
28 XXVIII
29 XXIX
30 XXX
31 XXXI
32 XXXII
33 XXXIII
34 XXXIV
35 XXXV
36 XXXVI
37 XXXVII
39 XXXIX
38 XXXVIII
40 XL
41 XLI
29 XXIX
43 XLIII
44 LIV
45 XLV
46 XLVI
47 XLVII
48 XLVIII
49 XLIX
50 L
51 LI
52 LII
53 LIII
54 LIV
55 LV
56 LVI
57 LVII
58 LVIII
59 LIX
60 LX
61 LXI
62 LXII
63 LXIII
64 LXIV
65 LXV
66 LXVI
67 LXVII
68 LXVIII
69 LXIX
70 LXX
71 LXXI
72 LXXII
73 LXXIII
74 LXXIV
75 LXXV
76 LXXVI
77 LXXVII
78 LXXVIII
79 LXXIX
80 LXXX
81 LXXXI
82 LXXXII
83 LXXXIII
84 LXXXIV
85 LXXXV
86 LXXXVI
87 LXXXVII
88 LXXXVIII
89 LXXXIX
90 XC
91 XCI
92 XCII
93 XCIII
94 XCIV
95 XCV
96 XCVI
97 XCVII
98 XCVIII
99 XCIX
100 C
101 CI